Because tap water including well water often contains chlorine, chloramines and heavy metals, that if left untreated can cause damage to the pond ecosystem. Aquascape Pond Detoxifier eliminates chlorine, chloramines, ammonia and chelates heavy metals making them safe for fish and plants and allowing for metals like iron to be used by pond plants as fertilizer. Why buy two separate products when Aquascape Pond Detoxifier can do it all?