You need fertilizers when the plants are growing faster than they can get nutrients from other sources in the tank, and conversely, the plants can't grow faster unless they get extra nutrients beyond what is already available in the tank. The thing that drives how fast plants grow is light. So, below a certain light intensity, the substrate and fish poop can supply all the nutrients the plants need, and adding fertilizers won't do anything much for the plants. Above that light intensity, you do need to add some fertilizers or the plants will be stunted - growing slower than the light is driving them to grow. That leads to algae starting to grow.
I'm not sure how much light you have, because I don't know either the dimensions of the tank nor what kind of light you are using - T5HO, T5NO, PC, T8, etc.