pH please have patience

Originally posted by TCLynx on AGC, June 15, 2010

We are talking about aquaponics which is supposed to be a living ecosystem, it is not a lab test kit. I have lately heard of several occasions where people used drastic measures to adjust pH and some of the situations were not so good for fish health.

Aquaponics is not like Hydroponics where you can tweak and adjust chemically. Any sudden changes can harm the fish and bio-filter and are to be avoided where possible.

First, an aquaponics system with time will naturally have a descending pH. The chemistry involved with the bio-filter and other bacteria working in the system will have an acidifying effect over time. This is why people usually have to add a buffer like shell grit to their systems as they become mature. (The systems that is, I don’t care how mature the people are.) Since systems will naturally drop in pH over time, it is usually not necessary to adjust the pH of the water in a new system if it is starting out with a pH between 7 and 8.

Second, the bio-filter might prefer the pH higher but the plants want it a little lower. Everything in Aquaponics is a balancing act, pH is no exception. It is probably best to see where your system pH wishes to settle once the system is cycled up and not fight it. If your top up water is very hard, your system may settle a bit higher, if your top up water is acidic, you will probably need quite a bit more buffer in the system to keep the pH from dropping off the chart.

So what is the optimum pH? As always, that Depends. But, I would venture that you want to keep your pH in a range that you can easily read accurately. By that I mean if you are using a pH test kit that can only read down to 6, then you want to keep your pH high enough above 6 that you can easily tell if your pH starts dropping and needs more buffer. Many people aim for a pH of 6.8 and they will add buffering material (like shell grit or lime) when the pH drops down to 6.5.

My situation is on the other side of the scale. My well water is from a limestone aquifer and after allowing the CO2 to escape it will test out between 8 and 8.2 or so. And then 40% of the media in my main system is shells. The shells naturally buffer to 7.6. My pH in that system is pretty darn solid and will be till all my shells dissolve. I have seen the pH in that system drop as low as 7.4 when there is a heavy fish load that is eating well and I supplement some Iron. Because of the high pH, I do regularly supplement Iron. Some plants don’t grow as well but many still grow just fine.

I have found no good way to bring down the pH of a system with a strong buffer as media. Adding acid will only cause the pH of the system to bounce. You can add acid and bring down the pH but by the next morning it will be right back up to where it was before. The acid simply causes the buffer to dissolve faster to bring the pH back up. A heavy bio-load is the only thing I’ve seen bring the pH down and that only works as long as the load is heavy, as soon as feeding slacks off (like from the weather cooling off) the pH will be back up to normal.

If you do have high pH tap water but your system has cycled and settled into a lower pH nicely, you might consider adjusting the pH of the tap water before adding it to the system when you need to do large water changes. Make regular top ups small and often to avoid spiking the pH too much.

The basic rules I work with for pH in AP.
Buffers bring pH up or keep pH up. Shells and shell grit generally buffer to around 7.6 most everything else buffers higher.
If the pH doesn’t drop make sure the media isn’t limestone.
Adding acid won’t do any good if the media is the buffer.
Acid is only useful to adjust the initial water but generally that isn’t needed.
Have some buffer material on hand because the pH will drop eventually.

If pH seems to be doing something strange, take into account a few things.
1-tap water will have a false low pH reading right when you draw it but if you bubble it for a night the reading will be more accurate.
2-Algae can affect pH with diurnal shifts. If algae is messing with things, the pH will be lowest before dawn and higher in the late afternoon. If you notice these kind of swinging going on, shade the tanks to get rid of the algae.
3-Keep an eye on systems that have passed initial cycling but didn’t have their pH fall right at the end of cycling, that pH fall will often happen some time later if there is no buffer in the system. Sudden unexplained ammonia spikes in well cycled systems are often related to a sudden pH drop due to the bacteria having used up all the buffering that was in the water.

If for some reason you must adjust the pH of an AP system, please do it very slowly in small increments.

39 comments to pH please have patience

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  • Jonathan Lopez

    Hello, I want to thankyou for sharing about ph, in AP, I have been dealing with my ph for awhile, not sure how to keep it at the same, alway’s….I’ve used sea rock’s, for sand, I try the best to keep it balances…but some how not sure what else that I should do…however rite now my ph for my fish tank is 6.2 or 6.8 would that rage harm the fish.

  • Mike


    Just checked my Ph from my well, which is 925 feet deep. It is in the 8.2-8.4 range. Have had my system running about 10 days, (with very few fish yet) and the Ph has not changed. Plan on waiting a few more weeks to see what it does. Any other things that I should be doing? Thanks…. Mike

    • TCLynx

      pH is not likely to drop much unless the bio-filter is actually cycling up and working hard. You should be testing the ammonia, nitrite and nitrate as well as pH to keep an eye on how your system is cycling up. Cycling up usually involves the ammonia spiking for a time then it will drop as the nitrite spikes (both ammonia and nitrite are dangerous to fish) as the nitrite drops that is often the first time you are likely to see the pH drop but if your well water is really full of calcium carbonate, it may take much longer to see the pH really drop and it will depend on how much well water you are having to use to top up regularly. (make sure your media isn’t keeping your pH high too.)

  • Matt

    My AP system is about 6 weeks old and the cycling is complete, ammonia, nitrites, and nitrates are at zero or in the healthy range. However the PH stays between 8 and 8.4. I do not have any limestone or similar buffers in the system and have been adding a PH down that I purchased from the local Hydroponics shop. I add it every morning because it works for the day but is back up each morning. I have a 30g tank and about 15 one inch fish, should I add more fish? I wasn’t going to because when the fish reach maximum size it may be too much for the system. My veggies are not growing but holding steady at current size….well I take that back my cilantro has taken off with ridiculous speed and is thick like a tree instead of how I see it in the grocery store, lol.

    • TCLynx

      What is in the pH down?
      What is the Hardness of your source water?
      Have you bubbled a sample of your source water overnight and then re-checked it’s pH?

      Adding acid (pH down) directly to your constantly may not be a very good way to deal with hard tap water. I would recommend that you adjust the pH of your top up water in a separate container and let it stabilize before using it in your system and this should slowly bring your pH down and be better for your fish/bacteria and plants than dumping pH down into the system every day and having the pH bounce due to the carbonate buffers in your tap water.

  • Noel

    Dear TCLynx,
    Just want to know my AP ph keep going down from 7, 6.8,6.4,6. but he fishs still alive an bright. what should i do? still add the Baking soda when its go down? whats makes like this? before everything normal. my AP at the out door, and in HK country side.

    thanks a lot

    • TCLynx

      The bacterial action in aquaponics that changes the ammonia and nitrite into the less toxic Nitrate for the plants actually causes the pH to go down over time. This is why we regularly need to add small amounts of buffer to our systems to raise the pH back up and keep it from falling too fast.
      I don’t really recommend baking soda since that will add more sodium to your system than your plants want.
      For gentle buffers that provide potassium and calcium I would recommend potassium bicarbonate and calcium carbonate. Be sure to only use small quantities at a time since you don’t want to raise your pH much, just keep it from falling below about 6.2. For a stronger way to raise the alkalinity of the water, some people will use potassium and calcium hydroxide. These are very strong caustic substances and can change pH rapidly and need to be handled with care so I don’t usually recommend them for backyard growing.

      A simple way many people will add calcium carbonate to their system is to put some shell grit or limestone chips into a mesh bag or some stockings and hang them in the tank. this way if the pH gets too high, you can pull them out for a while till the pH starts to drop again. Potassium bicarbonate can be found a beer and wine making suppliers and can also be used mixed with water to spray on plants to combat certain molds and mildews.

      Many people don’t experience the pH drop if they are using hard tap water or well water to fill their systems, they are likely getting lots of calcium carbonate in their water. People who use rain water or have soft water need to keep a closer eye on their system pH.

  • PT

    Hi TClynx
    My system pH is around 8.6 what should I do to bring it down. Secondly my tap water pH is 7.5 and when I do the aeration the pHgoes to 8.6!!! I don’t understand what’s going on? Could you pls let me know what can be done to bring the pH down so that I don’t have an issue of nutrient lockout.

    • TCLynx

      As to What is happening when you aerate the water Check out this link

      What to do to bring pH down, If the system doesn’t have fish or bacteria or plants living in it yet. For instance if you just filled it up, you can use acid to bring the pH down. Something like Hydrochloric acid HCl (Muratic acid) can be used, Or possibly phosphoric acid or even sulfuric acid (though I wouldn’t want to use too much of that regularly.)
      It may take several doses of acid (be careful handling acid, wear appropriate safety gear and never add water to acid, always add acid to water) to use up all the hardness in that water.

      Or if you already have fish, bacteria or plants in the system, you should probably do pH adjustments to your top up or change out water so as not to bounce the system pH and kill your creatures. This is a very slow process since you need to start with a measured amount of water and an aerator. Add some acid and let it mix, check pH, wait and aerate overnight, check pH, add acid again, let mix, check pH, wait/aerate overnight etc you continue this process until the pH is the same after you check it and the next day. Be sure to keep track of how much acid is used in total for that given amount of water in order to use up the hardness and get the pH to settle somewhere between 6.5-7.2 and stay there for a day. Once you find that out, you can calculate roughly how much of that type of acid is needed to counteract the hardness in a given amount of your tap water so the next time you need to prep tap water it won’t take so long.

      I don’t really recommend doing this (using acids) all the time or long term (since it can cause a build up of Chloride or phosphate or sulfur) if your source water is really so hard it might be worth while to look into RO filtration or if you are in a location where you can collect good quality rain water, that might be a good way to lower your system pH.

      • PT

        Thanks TCL .. Will share what we found after water testing analysis. Is there any method to provide nutrients during high pH values as plants have started turning yellow and not growing much

        • TCLynx

          For Iron Lock out, you can use Chelated Iron. For your pH levels (which I would recommend trying to bring down at least a little bit) you probably need to find EDDHA Iron Chelate to be most effective at your extremely high pH. If you can get the pH down below 8 (better yet 7.6) you can get away with using DTPA Iron Chelate which is a bit less costly than the EDDHA. I DO NOT recommend EDTA chelated iron even though it tends to be ceaper since it won’t be effective at providing iron at a pH above 7 and if your pH is below 7 you probably won’t need much iron anyway.

  • Eric Schmidt

    My system is medium sized – 2 IBC totes as fish tanks, 2 55 gallon drums as clarifier and demineralizer for each tote. My raft bed is 20 feet long, 6 feet wide, and roughly a food deep. I recently added water to my system and tested it. pH is ~8.4! This kind of freaked me out as I know this is higher than desired. All our water comes from a well, and this is likely entirely the cause.

    Do I need to drive my pH down in order to allow the plants/bacteria/fish to live/grow in it? I already used a gallon of pH down and it hasn’t really changed anything (pH is still 8.4).

    Thanks for any help you can provide

    • TCLynx

      When you say 2 55 gallon drums as clarifier and demineralizer, are you talking about a net tank sort of filter as the demineralizer? If so, how often do you clean that? If you have denitrification going on (as in anaerobic demineralization of the solids in a filter) that could also be driving your pH up.
      To safely bring down your pH if it is all totally due to your well water, you will need to prep your well water with acid before you use it in the system. It is best not to dump acid directly into an operating system since that can cause very harmful pH bouncing. Adjust the pH of your top up water and let it stabilize for a day before putting it in your system.
      If the problem is due to solids building up in the system and going anaerobic, then you need to do a filter cleaning and possibly also clean out the bottom of the raft bed (generally done yearly.)
      If you have any matts of algae growing anywhere, they can also be a place where things are getting anaerobic and causing the pH to go up.
      If you have an algae bloom going on you might find that the pH is low in the morning and high in the late afternoon.

  • Dreen

    Hello TCLynx,

    Started my system with combo raft, lava rock and expanded clay beds. Never have gotten my ph to settle, and even though I checked the ph on one of the bags of lava rocks and it showed 7.4, after fighting this for months, I pulled lava, let it soak overnight, and the water measured 8.4. So, joy of joys, I separated the Grow it pebbles from the lava rock and pulled out 20 bags of that by hand.

    So, now I have two cut in half barrels of Grow it balls and the rest are rafts. The water is nice and clear, but I am running high 160 on nitrates, ph is at 8.2, despite adding top off water at 6, and tap (well water) at 7.6 ph. I am also having nitrites showing at .25 while ammonia remains at zero.

    FWIW, the system was started in December so it should be pretty well settled.

    Any advice?

    • TCLynx

      When you check your well water pH, are you making sure to bubble it for a day before you test it? There is a thing about water out of the pipes or the ground, there is CO2 trapped in it and if you check the pH right out of the tap or hose, that trapped CO2 is acting as a weak acid and giving you a false low pH reading. The actual pH reading could be much much higher.
      My well water if I check the pH just a few minutes after filling a cup of water from the hose or faucet, it will read a pH of 7, but if I let a bucket of this water bubble for a day and then check the pH is is going to read over 8.
      Were you soaking the Lava rock in distilled water or in your tap water to check it’s pH?
      If you have really hard tap water, then that is likely the culprit causing your pH problems. How are you getting top off water adjusted down to 6?
      Now if your well water is not the culprit, the other thing that can cause a high pH is anaerobic activity in your system. If there are solids building up anywhere (or even big matts of algae) that could be causing an elevated pH instead of a dropping pH. If there are solids starting to accumulate in the raft beds or if there are any solids filters that are not getting cleaned frequently enough that can cause you problems and if there are high nitrates in the system, anaerobic or anoxic conditions can cause denitrification which converts nitrates back to nitrite which can be bad unless it is offgassed before it gets back to fish.

  • Dreen

    Thank you….I have a little sludge, but not any algae mats. I clean out my filters, nylons, scrubbie pads on the bed filling waterers, every day. On the drain lines from the tank and beds, often twice per day. The water feels very soft though, if that makes sense.

    I checked the lava rock in my tap water. My water reads 7.6 straight out of the hose, and 8.2 (which is where it is sitting in the AP system) after bubbling with an aerator over night.

    I am adding a five gallon bucket at a time adjusted with muriatic acid.

    • TCLynx

      what is your procedure for adjusting the tap water with acid?
      If you simply add some acid and mix then check the pH and add to system all in once swift operation, then you may not be adjusting enough. I would say you add acid then let it mix and bubble for a day and check the pH to make sure that it has actually settled. Often after adding acid into some water the pH will come down but then after several hours the carbonates will dissolve and bring the pH back up in what is often known as pH bouncing (also what happens if you add acid into a system with lots of carbonates.)

  • Dreen

    I’ve just let it set overnight. Should I aerate it as well to release the carbonates? I even saved a bucket of rainwater and without aeration it was 7.4 Just checked it after collection and thought if it was that high there wasn’t much point in saving the rainwater for addition to to the ap system.

    • TCLynx

      You might want to look into getting some other test kits to help you pin down what the actual problem is. These would be basic aquarium type kits for a carbonate hardness test kit could help you sort out how much buffering capacity your water has. I suspect that rain water, though it might have a pH a little higher than you like, won’t have much in the way of carbonates and therefore the pH will drop easier than the tap water.
      So with the tap water, you are adding acid and then just leaving it sit overnight? The what is the pH in the morning when you test? I would recommend bubbling it just to aid in the proper mixing of the water with the acid to make sure you are actually getting an accurate reading.

  • Dreen

    When I add it in the morning it is at a ph of 6. But that isn’t aerated, so that is probably the issue.

    Do you think having more media would be helpful?

    I have some breeders in my sump, and I could put them into the fish tank again and isolate the tank and adjust the ph in the whole system except the tank, then slowly add the ph adjusted water to the fish tank. Or is that flirting with disaster with my fish?

    At this point, I have horrible growth of plants, and spend entirely too much time on this. Just hoping to harvest the fish and then reevaluate everything about the entire AP system. It’s very upsetting because I have always been a good gardener, and this is incredibly humbling. And it’s very inefficient and probably all because of hard water.

    • TCLynx

      It probably is flirting with disaster to mess with acid directly in the system. I suggest that you should get a carbonate hardness test kit so you can measure that in your system as well as the tap water to ģet a handle on where the issue is coming from.

  • Dreen

    Okay. I’ll order a kit. Then is there any way to deal with it other than reverse osmosis? I can collect rainwater, but that would actually be something that would take time to do here. I don’t have a large system for catching rain water. Even with that I need to check for carbonates.

    So what about additional media? Would that be helpful at all?

    • TCLynx

      IF the tap water is the problem (and I suspect it is) then reverse osmosis is probably the easiest way to deal with it in a location without ample rain to collect. Now you may not need to use 100% RO water, it fact you probably wouldn’t want to do that but to get the problem under control you may need to change out a large % of your water with RO water and then you might be able to change to just topping up part with RO and perhaps use some straight tap water when you need to buffer the pH back up.
      If your ammonia is 0 then I suspect you are not hurting for more media at the moment. However, I don’t know much about your system.
      How big is fish tank, how much media do you have, how many fish/what size, how much raft, how much aeration etc. Are there any gunky spots forming in the media beds? Are the media beds flood and drain? If there are anaerobic areas, that can elevate pH. Additional aeration may help with your slightly elevated nitrites. Look up Salt for Fish health if the nitrites are getting over 0.5 ppm.

  • Dreen

    I have mostly rafts due to the expense of the media. It’s a CHOP system with a 210 gallon tank and 11 beds. I have clay pellets in the grow cups in the rafts (9) and two half barrels full of clay pellets, and they are flood and drain. It seems like I have pretty good aeration. In the tank I have an aerator, and the beds and the tank drop into the sump at a good rate of flow.

    Will look up the salt info. Thank you!

  • Dreen

    Sorry, they are cut in half 55 gallon barrels. I figure about 22 gallons of water is the average for each. I figure it is about 500 gallons all told with the sump, beds and tank.

    • TCLynx

      Keep in mind rules of thumb that for backyard type systems, only stock one fish per cubic foot of media bed and maybe one fish per 3 square feet of raft bed (provided you are pumping air in under the rafts).
      So you have about 100 gallons of flood and drain media bed, that is good to support about 13 fish growing out to 1 lb each.
      The rafts, well about 3 square feet of raft bed might provide nitrification for another fish. And A half barrel is about 6 square feet of raft.
      So hopefully you only have about 30 fish???

      Just so you know, your lava rock probably was not causing you the pH problems so if you wanted to add lava back into some beds to provide more filtration, you could.

      Once you have the carbonate hardness test kit, you will better be able to adjust the buffering of your water so that the pH may be able to come down.

  • Dreen

    I have about 35 fish. Several of them are at about a pound already. Some may be over that, but it wouldn’t be much. Since the ammonia has been very steady, I didn’t think I was overstocked really.

    Repurposed the lava rock already, and I don’t think I want to chance putting more of it back in. I got the ph at 7.8 this am, and it went up to 8.0 this pm. Very hot day, and I think that may a factor in raising.

    I have two beds out of rotation and could put them back in as rafts, but it seems like I really need more media…and that is what my gut has been telling me.

    You know the things they use for vertical aquaponics towers? I am pretty certain it is a material that I can get that works out to less than the cost of the leca balls. Do you think it would be okay to do some barrels with that on the bottom and the leca on top? About half and half?

    • TCLynx

      Perhaps you could but I don’t think sticking the filter media under loose clay or gravel type media is really a good option. If you are seeing a swing of pH between dawn and late afternoon, that pH swinging could be due to algae. Algae will use up O2 and give off CO2 overnight and the CO2 in the water acts as a weak acid and gives you the lower pH reading. This usually isn’t a big issue unless you have a lot of green water or matts of algae somewhere.

  • Dreen

    Finally have some KH readings. On the AP system, in dKH it is 7.0. The water from the air conditioner (so out of the air) is 7.7 in dKH. I tried to find info on what I need to do in relation to these results, but can’t readily find anything. The system has been staying pretty steady at 7.8 on Ph.


    Thanks so much for all you do!

    • TCLynx

      what is dKH? Does that mean drops? Do the instructions have a coversion that tells you what the ppm is? For instance my test kit instructions tell me that 7 drops is equal to 125.3 ppm. If that is the case with your kit, then that tells me that there is a lot of carbonates or buffering in your water and that your use of acid has not been totally dissolving the carbonates before you add the water to your system.
      A KH reading of around 40-50 ppm is adequate to supply the bacteria with the alkalinity it needs to function and will allow your pH to come down more.

      So, you probably need to do a more through pH adjustment on your top up water before using it in the system. Set up a barrel or trash can with your tap water and an aerator to keep it mixing. Add a measured amount of acid to the water and let it mix overnight, check the pH and KH the next day. If it is down at the desired level then let it aerate another day and check again to make sure it stayed stable. (If you overshoot the amount of acid and the pH is below 6 then you need to start over.) If it is stable after the second day then remember the amount of water and acid used to get that result. If it didn’t reach the desired level or stay stable after the second day, you add more acid (keep track so you will know the total) and let it aerate for a day, check to make sure you reach your target levels, if so aerate a second day to make sure it stays stable. Etc.

      Yes this is a very tedious and time consuming operation. Once you have figured out how much acid it takes to bring the pH and KH down in a measured amount of water you will then have a baseline of how much acid it will take to adjust a given amount of water and the whole operation should take only the two days (in general, of course there are fluctuations in water supply so you should still double check before using the water.)

      Basically you want to reach a pH just above 6 and you want the KH Low (while you are trying to get the hardness down in your system you will be fine adding water without any hardness but once your system KH is down below 50 you will want to make sure the system hardness never drops to 0 because then the bacteria could crash.)

  • Dreen

    I was hoping you would know! The instructions are evidently translated from German and not the most clear instructions.

    The instructions tell me to add reagent from a 1ml syringe to 4ml water and 4 drops of KH-IND. Add one drop of reagent to the vial and swirl in between every drop. When it changes from blue to pinkish, look on the 1ml syringe and the level of reagent left is your dKH. Then the alkalinity is given next to that number, and that is in meg/L.

    The corresponding alkalinity reading is 2.50 for the aquaponics system and 2.73 for the air conditioner water. The a/c water measures at 7.4 ph, so I was thinking I would simply use that for the top off as it is 7.8 out of the tap without aeration. I

    Will follow your suggestion for the top off water and make certain it is stable before adding it to the system.

    The test kit I got is a Salifert Profi-Test for alkalinity and hardness.

    • TCLynx

      Condensate dripping from an AC unit is NOT really the safest thing to use for anything. While the water condensing is basically distilled water, the dirt/grime/microbes etc on the fins and in the pan ARE NOT what you want to be putting into a food system without treatment. (legionnaires disease has at times been traced back to contamination from condensate of air conditioning units.)

      Even if we are not entirely certain what the reading from your test kit means, you can still use it to figure out how much acid it takes to get your tap water to read 0 alkalinity. Remember to work to that point slowly since if you pass that point there is no way of knowing what level of acidity you reach.

      Then for a period of time you top up or do small water changes with your system water until your system water alkalinity starts to drop. You don’t want your system water alkalinity to drop to 0 though, the bacteria need a little bit of alkalinity to work. Once your system gets down to a lower level of alkalinity and the pH comes down a little, you will need to allow some alkalinity to get back into your system from the water or you will wind up needing to buffer the system a bit.

  • Dreen

    Okay. Good point on the ac water. Maybe I will just distill water and work up from that. I am not certain if distilled water from the store is from a fluoridated/chlorinated source, and that might give me more issues if it is from treated water. I don’t need any more problems!

    Thanks for your help!

  • jason

    im using a 1000lt ibc with 3 200lt barrel filter system. barrel 1 fallout remover, barrel 2 moving bed bio filter with 80lts k1 media, barrel 3 fines filter and sump. in my sump i have about 8kgs of dolomite rocks and about 10kgs of shell grit. my ph is always falling i like to try keep it just over 7 to 7.5. my KH is 1DKH sometimes zero? even though i have all this carbonate media is does not appear to raise my KH? ive resorted to adding carbonate hardness generator by aquasonic (this only seems to raise my ph and get my kh up to 1 again. for the life of me i cant work out why my kh is rock solid on 1 or below? anyone any ideas? my fish load is 40 Australian silver perch about 200mm long.

    • TCLynx

      What is your source water? If you are using a source water with no hardness to begin with, then you will need to be adding those minerals back into the water constantly at a rate to balance the acidifying effect of the bio filter bacteria. Where I’m at, my well water is a bit like topping up with liquid Limestone so I generally didn’t have that problem in my system. Now people who are using RO water or rain water are going to have to put considerable effort into making sure there is enough calcium, magnesium and carbonates to keep everything in balance.

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