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.

13 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.

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