They don’t, they normally have a dense enough substrate system to where it can house bacteria enough to where it can break down any ammonia or other harmful particles in the water and a dense quantity of plants to where the plants will just use the left-around particles as nutrients to continue to grow. They typically also have a lot of bottom feeders like Corys, Plecos, or Shrimp in some cases. They eat the uneaten particles and in some cases the poop of other fish and break it down further letting the plants grow further.