Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Diana Walstad's book Ecology of the Planted Aquarium goes into this in much greater depth. Here are a few basics:
Plants use most. These are not thought of as fertilizers:
Hydrogen (H), Oxygen (O), Carbon (C).
We can supply carbon in the form of carbon dioxide (best), or liquid carbon products (work well for many plants, not all). Some plants (roughly half the plants we grow in aquariums) can get the carbon out of carbonates.
Fertilizers plants use the most. These are called macros:
In a packed fertilizer the three numbers on the package are the % by weight of these three materials.
Fish food has a fair amount of N and P. It does not have much K.
We can add fertilizer with these elements such as KNO3 (supplies potassium and nitrogen), KH2PO4 (dosed in low levels, so that although it has some potassium it does not count for much) K2SO4 (supplies potassium and sulfur).
Fertilizers used at rates somewhat less than the macros, these are secondary nutrients:
Ca and Mg can be supplied by the water if the GH of the water is over about 3 German degrees of hardness. Plants use these 2 in a ratio of about 4 parts Ca: 1 part Mg. If there is any doubt about there being enough of either of these, they can be found separately (Calcium chloride, magnesium sulfate and other materials) or in combination such as GH booster. Usually fish have a specific range of GH where they thrive. I would make the water suit the fish while maintaining a 4:1 ratio between Ca and Mg.
Fish food is somewhat low in Ca and Mg.
Sulfur is a common ingredient in most fertilizers. Many of the minerals in fertilizers are in the sulfate form, such as potassium sulfate or magnesium sulfate.
Minerals used in lower levels, referred to as trace or micros.
Iron (Fe) is the one that many people will add separately. Usually a chelated form. Chelation means the mineral is locked up in a molecule that plants can get into, so the mineral remains available to the plants, but is not available to get into combination with certain other minerals.
Fish food is somewhat low in iron.
All other minerals (roughly a dozen) are used in such small amounts they are generally lumped together. Fish food usually has enough of all of these.
In a low tech tank, or high stocking levels, where you are adding a lot of fish food and the plants are not using up the nutrients from fish food (digested by the fish, or rotting in the substrate) the test results before adding any fertilizers will show rising NO3 which you do water changes to keep low. Under these conditions you may assume the P and traces are just fine, too.
If the GH is OK from the water, then I would simply dose carbon, potassium and iron. And keep up with the water changes.
In a high tech tank, or low stocking levels, where you are not adding very much fish food, and the plants are pretty much using up all the nutrients from fish food, then the NO3 test will show it does not rise very much, and may simply maintain 0ppm.
Under these conditions you may assume that the plants are also going to be needing all the other nutrients.
The Estimative Index is one method of supplying all the nutrients that plants need in the ratios they use. It is based on supplying all the nutrients in a slight excess, then doing a big water change once a week to remove the excess.
PPS-pro is another method using similar ratios of each nutrient, but at much lower levels, not demanding a big water change.
Seachem makes a complete line of fertilizers with each bottled separately so you can customize the dosing for your tank.
There are other methods, too.