there is a magazine out now called marine fish and reef 2004 annual (picture of a lionfish on the front) at your local petstore with an article on live rock hitchhikers. it covers the isopods cirolanid and aegid. they multiply quickly and can overtake your tank killing all your fish. to prevent this happening, when you get new live rock you need to dip it in saltwater with a specific gravity of 1.035 for a minute (not too long!) and the hitchikers will bail off. the article says, "If you see one in your system, look for more. They are seldom seen simply swimming or crawling on the bottom of the tank: they are usually noticed on a fish, where they are seen fastened to the fins or body. The easiest way to remove them from your system is to catch the affected fish and use a pair of forceps or tweezers to remove the isopid from its body. Afterward put the fish into a hospital tank with some antibiotics, and leave it there until the lesions left by the idopod have healed. Sounds eay, right? Wrong!" it goes on to summarize:
the isopod may abandon the fish while youre trying to catch it. Once it leaves the fish, it will be impossible to catch. A female cirolanid can be carrying 30-50 juveniles. most isopods are fully nocturnal. they feed on fish at night and bail about an hour or two before dawn. you essentially have to remove all fish from the display tank and let the little bugs starve. the bad news is that it takes between three to six months. the article says after 6 mos, put a potentially sacrificial fish (usu. damsel) in the tank and watch it closely for several days to ensure it is not being eaten. If not, then add your fish slowly back into the display.
If I were you, I would keep a CLOSE eye on your other fish. If you see another one of these on your fish (get up in the middle of the night to see if there's any on your fish) then you need to probably take more drastic measures. good luck!
sorry about your loss. and good luck in the future lets hope those critters dont com back.