In addition to the regular monitoring steps, there are additional steps you will want to take if you notice a sudden change in the aquarium or anything else concerning. If you are certain that there is a problem, refer to the emergency section
Observe all fish closely signs of disease : If one fish is obviously sick, look very closely at all of your fish. Even behavior changes such as not eating or not coming out as often can be signs of problems.
Water changes of up to 50% can be instituted every other day with only 10% of water being returned to the tank hourly as to not shock a system. This is more accommodated by fish than corals and invertebrates however should be manageable if done to relieve water quality issues in a crashing tank. Remember if there is any smelly rocks or questionably dying corals remove them to a separate tank/bin for monitoring. If the sand is very dirty and when moved clouds of brown are released, gravel vacuum as you do the water change.
Check the filter. It can be clogged or not running at all (from losing prime , breaking the impeller, or burning out the pump). Make sure that your sponges and other filter media are not saturated with gunk, if so rinse them out with salt water if possible.
Tests for Calcium and Magnesium. Read more about this in the Water Chemistry section