So what do you do if you need to take the pH down in your aquaponics system? To lower pH we recommend certain acids, such as nitric, muriatic, and phosphoric. Our preference is phosphoric (AquaDown) because it is the safest of the three acids (it is an ingredient in cola drinks) and it adds some phosphate into your system, which your plants will like. That said, phosphates can exacerbate an existing algae problem, so if that is your situation you may want to use one of the other types.
You should absolutely avoid citric acid because it is anti-bacterial. Also avoid vinegar because it is too weak of an acid – you could be pickling your fish before you see the pH results you are looking for.
Rapid changes in pH can be harmful to fish so use sparingly and adjust over several hours or days. Add to system a little at a time, wait for it to distribute throughout the system, then retest pH. Repeat, until the desired pH is reached.
The quantity necessary to lower your pH will vary depending on the nature of the grow media and the carbonate level of the water in your system. To find out how much to use in your system we recommend a technique called titration. Take 1 gallon of water from your tank, figure out how much AquaDown you need to decrease the pH to the target level, and then multiply that by the number of gallons in your system.