Lets start with the tank itself. The lighting system is the next important area to consider. Filtration is of course critical. Additives are always a tricky thing to put your finger on. How you maintain your aquarium is also important.
Water chemistry is a critical parameter.
Iodide and strontium levels are also useful to know but not critical. One of the best ways to help yourself understand what may be happening in your aquarium is to keep a logbook. Each day write down anything you do to or observe in the tank. Record all your water quality test results. Often, when looking back in your logs you may be able to trace a change in your tank to something that happened or you did days, weeks to even months ago.