The problem is either one that has built up over time and is just now catching up to your livestock, or one that has been recently introduced.
If the problem is one that has built up over time, I can think of poor tank maintenance, dirty (and not performing optimally) skimmer and/or powerheads, chemistry out of whack. That said, I would suspect if it was poor tank maintenance, both fish and corals would be suffering, but it's something else to look at.
Any new additions to the tank lately, either fish, corals, LR, sandbed, new food, or anything else that could have introduced disease or parasites?
If the problem is parasitic isopods, my understanding is that some species burrow right into the fish and may not be obvious when examining the dead fish other than an initial hole and a deflated area under the skin where the isopod has eaten the fish. One article says isopods are most active an hour or two just after lights out and before lights on. These times, especially after lights out, would be a good time to look at your fish using the red light as described in one of the links. If the problem is other types of parasites, I would imagine the same methods of looking for them would also apply.
BTW, any new fish deaths over the last day or so?
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