Aquarium water quality can start to take a turn for the worst for any one of a dozens of reasons. Phosphate could be leaching out of the live rock, the sand bed could be a source of nitrate, the filters on the RO/DI unit could be exhausted, and so on. The reasons could almost go on forever, but if you drill down a little further into why these things happen, the two most common reasons are because the hobbyist is either two busy or too lazy. When either of these two occur, the maintenance tasks just don’t happen. Algae and other undesirable critters overtake the aquarium and corals start to fade away into obscurity. If you get into that rut, which we all do at some point, there’s no real easy way to get things back to shape other than to just put your nose to the grindstone and get it done. Make things as simple as possible for yourself, and make sure to perform those water changes.