Plants need (N)Nitrogen, (P)Phosphorous, (K)Potassium, and several other elements, but in extremely small amounts. The first three are called "macro" nutrients, and the last are called "micro" nutrients, or trace elements. The easy way to dose these nutrients is to dose potassium nitrate, for potassium and nitrogen, and mono potassium phosphate, for phosphorus and a tiny bit of potassium. Then add a "trace element mix" such as Flourish or CSM+B, and you are providing all of the nutrients plants need.
Plants need carbon more than any other nutrient, but that is rarely considered to be a fertilizer, since terrestrial plants use atmospheric carbon dioxide for carbon. We inject carbon dioxide into the water to provide the carbon.
There are a few elements that are in almost all tap water, like calcium and magnesium, which plants need small amounts of too. If your tap water is very soft, containing very little of either of those, you can use a GH builder, like Seachem Equilibrium, to provide that for the plants.