Other than CO2 water and light, All plants need the following nutrients to grow:
Nitrogen, Potassium, Calcium, Magnesium, phosphate, sulfur, chlorine (very smount in the form of a safe chloride salt), iron, Manganese, Boron, Zinc, Copper, molybdenum, and Nickel.
If just one of any of thos eis missing plants will not grow and eventually will die. Even easy plants light duckweed and mosses can be killed by alack of nutrients. Also Animals also need these same nutrients. But they get them mainly from their food. So if you have enough fish the waist from the fish may be enough to fertilize your plants. Other than fish waist plants can also extract nutrients in the water or in the substrate.
So if you use RO water with an inert substrate with no nutrient in it with few or no fish plants will not grow in the tank. In that case you have to use a fertilizer.