Not sure if you have seen a picture like this.
The idea is that plants need a whole range of mineral nutrients (plus light and water) to grow. They can only grow as much as the least available nutrient.
Your plants will be getting Nitrogen from the fish pee (basically) but most of the rest of the nutrients / minerals will be really low or not present in your water (represented by the barrel planks in the picture being shorter than others).
(good) Complete fertilisers contain all of the right minerals in the right ratios.
The idea is that plants need a whole range of mineral nutrients (plus light and water) to grow. They can only grow as much as the least available nutrient.
Your plants will be getting Nitrogen from the fish pee (basically) but most of the rest of the nutrients / minerals will be really low or not present in your water (represented by the barrel planks in the picture being shorter than others).
(good) Complete fertilisers contain all of the right minerals in the right ratios.