Fouling is the multitude of organisms growing on the fish farm nets. Overgrowth of fouling causes lower oxygen levels inside the cage of the water and brings conditions encouraging illness of the fish.
The fouling is, in fact, animals that have sting cells. These are poisonous compounds released as a defense mechanism. These compounds are released when heavy foiled nets are cleaned, releasing these compounds in the water; other types of life on the sea are reduced/destroyed, bringing a disbalance of the ecosystem.
Contact of those compounds with fish gills and the skin can heavily affect their health. Avoiding cleaning of heavily fouled nets will reduce this risk and drop tons of the gutted weight fish farms produce with ill fish.
The seabed below the cages has a limited capacity as a recipient. Copper (the compound in the anti-fouling used for the nets) is known to reduce to natural decomposition of feed spillage and feces from the fish. All flushing will also take off anti-fouling from the nets. This will fall to the bottom.