Plants tend to uptake what they need, as long as those nutrients are present.

The pH in an Aquaponics System needs to be between 7.0 and 7.5. This is the best balance, as the bacteria need the pH to be above 7.0 in order for them to do their job (read AP-101, tab at top of page).

The plants have a wide range of acceptable pH. DO (Dissolved Oxygen) is also important and needs to be at least 5 ppm or higher for the bacteria to perform well, and is also needed for the plants. Some plants, like leafy greens, need little DO, but not zero; while others need lots of DO, like tomatoes and peppers. Low levels of DO can cause root rot, a fungus that lives in moist conditions where there is a lack of oxygen.

Everything else seems to take care of itself. But, it is always good to add some Calcium (lime, if not already in your water supply), Potassium (we use potassium hydroxide) and Chelated Iron; otherwise your leafy greens may be leafy whites along with white cucumbers instead of green ones etc.

Yes, some plants require higher levels of certain nutrients, like tomatoes and other flowering plants need high levels of Nitrates when flowering. Just make certain that your nitrate level is adequate, 40 ppm for leafy greens, 80 ppm or higher for flowering plants; but once you reach 100 ppm you are starting to push the limit for the fish. Anything over about 150 ppm of Nitrates is toxic for the fish (Tilapia).

Your use of the term "pond" concerns me a little. We usually call where the fish live a fish tank. Pond implies a large hole in the ground filled with water, which can be used to hold the fish; but it is more difficult to control the chemistry due to the assumed openess and being subjected to the elements, as well as the ground beneath and what else might be growing in there. Aquaponics requires a medium density of fish to water ratio in order to generate enough nutrients to feed the plants. Ponds are generally populated with a low density of fish to water ratio.

By the way, Aquaponics is a sub-set of BioPonics. BioPonics is the process of converting organic material into plant nutrients and giving them to the plants by way of circulating water. In Aquaponics, fish are part of the conversion process, but with BioPonics fish are optional. Without the fish we have another sub-set of BioPonics called NitraPonics, the conversion of organic nutrients to nitrates using just the bacteria. Chew on that!

To be continued,

Oliver