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Plants need about a dozen elements to live.
Some of these are easy to supply, some more difficult.
Part of the concept of Low Tech is that you make the ferts a part of the set up so you do not have to be constantly adding them.

Plants need such basics as hydrogen and oxygen. I think in an aquarium, there is enough of these.

Plants use a lot of carbon, but in a low light situation there is enough CO2 entering the water from the air and from fish respiration. Adding some carbon in the form of Excel is a good way to go if you think you need to add some.

The next group is referred to as macros. These are the fertilizers plants use the most of.
Nitrogen. Fish food supplies enough N in a low tech tank.
Phosphorus. Fish food supplies enough P in a low tech tank.
Potassium. Fish food seems to lack enough K. The first deficiency symptom I noticed in my planted tanks was potassium deficiency. If you can supply this in the form of root tablets, the Safe-T-Sorb will help by holding the K in a way the plants can take it, and not let the K escape into the water.

The next group of fertilizers are the secondary ferts. Plants need these in amounts less than the macros, but more than the micros.
Calcium. Usually supplied by water, if the GH is over about 3 German degrees of hardness.
Magnesium. Usually supplied by water, if the GH is over about 3 German degrees of hardness.
If the GH is under 3 degrees, then I would add a GH booster at each water change, and monitor it in between. MAYBE need to add a bit more. Slow growing plants won't need much. The fish also rely on this as their source of Ca.
Iron. Fish food does not supply much iron. I would supplement, even a low tech tank. You can use certain materials in the substrate, root tabs or water column dosing. Eventually there will be a reserve of iron in the substrate to give you a bit of a cushion.

The last group is all the minerals that plants use in very small amounts. They are lumped together and called micros. Fish food probably supplies all these. Some root tablets have some of these minerals.

Fish Food = N, P, Micros.
Water Changes = Ca, Mg
Best to add = K, Fe
Good option to add = C (such as Excel)
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