Have you recently discovered a problem with your aquarium such as fish and corals suddenly not looking too well or even random livestock death? These things happen far too often, and most hobbyists have no clue why. To diagnose the problem, go back to the last thing you changed on your aquarium. Did you install new light bulbs? How about a new water pump? Did you start or stop dosing a particular additive? The tiniest deviation from your typical aquarium maintenance routine can cause all sorts of problems, depending of course on what that change is.
With all that being said, don’t be afraid to experiment. Trying out new techniques or products can lead to amazing results, so don’t let your tank fall into a comfortable rut.