Phosphate has earned a suspicious reputation in the aquarium hobby. When algae appears, phosphate is often marched into the interrogation room before the test kit has even been opened. The truth is less convenient: phosphate is not automatically bad. Plants, corals, algae, bacteria, and other living organisms require phosphorus. Trouble begins when phosphate accumulates beyond what the aquarium can use—or when it falls so low that plants and corals cannot grow properly.