Many aquaponic systems, as well as aquariums start off with a high pH. This is mainly because the tap water is buffered high in the alkaline range and refuses to drop lower in any meaningful way. In the aquarium hobby we can use a number of things to drop the pH over time. They include Indian Almond Leaves, Peat moss, alder cones and some acids, but I don’t use or recommend Lemon Juice to reduce pH.
Nate Storey of Bright Agrotech has done a video on using lemon juice to reduce pH where he also advises against it as well. I did see another video recently talking about using lemons and limes to control pH, and again lemon ju8ice to reduce pH was frowned upon, while lime juice was suggested as an alternative. It would seem that lime juice is less concentrated. Either way, I would tend to stay away from these two fruits because the active ingredient, citric acid is also anti-bacterial. The idea is to get the pH lower and have as powerful biological filter as possible. When you use lemon juice to reduce pH, the beneficial bacteria that are transforming ammonia/ammonium to nitrite and then to nitrate are inhibited.
Lemon Juice to Reduce pH Harms bacteria
Damage to the bacterial layer can result in a number of problems. Chances are that there will be spike of ammonia in the aquarium as the bacteria that reduce it rapidly are inhibited. Unless there is little buffer, the pH will create ammonia in the tank, which is toxic to both fish and plants. So the damage that can be caused by adding lemon juice to reduce pH can be substantial. It is also quite concdentrated, so unless you are treating tanks well above 250 gallons (not your standard inside display aquarium), you may want to look into the other methods of dropping pH as discussed in an earlier post entitled Adjusting pH.
Lemon juice to reduce pH will certainly do the job, but its side effects are so devastating that is is not a true alternative. Stick to tried and true methods on the aquarium industry to get it down, or follow the best advice, reduce carbonates with RO water and let it drop down over time. The fish and the plants will not have many problems this way.