Once you get through the first couple months of having your Aquaponic Setup your levels should be getting fairly consistent. It is still a good idea to check your pH, at least twice a week if not more. In addition to pH checks we also recommend checking Ammonia, Nitrite and Nitrate levels at least once a week. This will give you an indication of how your garden is doing, insures the safety of your fish and the prosperity of your plants.
Ideally you want your pH to be between 6.8-7. This is the happy medium between fish, plants and bacteria. Plants typically like a slightly acidic pH of 5.0-7.0, fish prefer a higher range of 6.5-8.0 and bacteria do well with a pH of 6.0-8.0. Know that a pH of 7 is neutral. Over time due to the natural nitrification process that is occurs in your mature system your pH will slowly drop. When your pH naturally drops the best way to raise it back up is by using a buffer. We like to use Calcium Carbonate and Potassium Carbonate (or bicarbonate) for our pH buffers. That leads us into our next subject, nutrients.
Plants require 13 essential mineral nutrients in order to thrive, the 13 nutrients are; nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, zinc, iron, copper, manganese, boron, molybdenum, and chlorine. Aquaponics naturally provides 10 out of these thirteen but is normally lacking in 3; Calcium, Potassium and Iron.
As we stated above we like to use the Calcium and Potassium buffers. By using these you are killing two birds with one stone by raising your pH (if needed) plus providing essential nutrients. Foliar feed with calcium and potassium if your pH is already in the high range (7 or higher). Start by adding a little at a time until you get the correct dosage rate for your particular system, then check your pH the next day. This is the safest method because it can take a day or two before your pH changes after adding calcium or potassium. Always keep in mind that large swings in pH (.3 or more) can negatively effect the health of your fish.
The other important nutrient that we need to provide is Iron. We use 11% DTPA Chelated Iron. In order to check the Iron levels in your system you must purchase a specialty iron checker. We purchase ours online at Hanna Instruments or at our local San Diego Hydroponics and Organics.