The correct answer is (b), only two of the statements are correct.
Biofilters provide waste treatment by removing uneaten fish feed and converting ammonia present in fish waste to nitrate. Biofilters do not increase phosphorus as a nutrient for fish in water.
Uneaten fish feed and fish waste produce ammonia, which is toxic to fish. Biofilters contain bacteria that convert ammonia to nitrate, which is less toxic to fish. The nitrate is then removed from the water by the filter media.
Phosphorus is a nutrient that is essential for fish growth. However, too much phosphorus can lead to algae blooms, which can suffocate fish. Biofilters do not remove phosphorus from the water, so it is important to monitor the phosphorus levels in the water and add a phosphate remover if necessary.