killer insect

Welcome to A.C. are you sure it's killing the fish? It sounds like it's probably a copepod and most likely it's just scavenging from the body. Are their any other symptoms like leisons on the skin or inflamation. What sort of fish do you have ? How long has your tank been set up and what are your water parmaters nitrates, ammonia etc? What do you have in your tank as far as live rock substraight etc? It would really help us provide you with better answers if you'd give us a bit more information.
I'm very sorry for your loss and wish you the best of luck.

What are your ammonia, nitrites and nitrates and water change schedule?

Kim

wow, you have a LOT of seriously LARGE fish....... even for roughly 225 gallons. ......






for what it's worth, I have a feeling you might not be aware of what copepods really are (maybe someone can post a pic?). they are a beneficial part of your reef system... most tanks have hundreds of these little critters (my little 20 gallon has many more than I can count). My guess is that your fish have Ich. This will cause them to rub and scrape against things in an attempt tp itch themselves. And in the worst case scenario you'll see sores and wounds on the fish from these attempts.....well...... now that I've written all that I'm confused as to how you would have copepods if you don't have any live rock? Do you have any corals or anything "live" that isn't a fish?

It's also possible that some of your fish don't like each other, hence the deaths...

Search on isopods. There are some isopods that are parasitic, but you will actually see them on the fish, in addition to being free-swimming in the water column, though usually only after the lights go out. If the critters you're seeing are not on LIVING fish, they probably are not the culprit, since non-parasitic pods are detrivores that will certainly consume a carcass.