If your tank is heavily planted, and lightly stocked, you may find fish waste is not a problem. But plants use chemical nutrients from solid waste that has already been broken down by a myriad of other animals.
In nature worms, crustaceans and molluscs, etc, break down waste into smaller pieces that are broken down then again, by rotifers, and smaller nematode worms, and other microscopic life.
In aquariums, fish tend to eat the macro animals that are the initial stage of waste breakdown.
Although some of the shrimp, and small nocturnal snails that live within the substrate, are available, and do a pretty good job of breaking it down for plant use.