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.