More information from Ron Shimek:
If you notice a cirolanid in your tank, and it leaves the fish, there is almost nothing that may be done to catch it. They are very fast, small, and quite capable of avoiding a net. And if it is a pregnant female (remember all isopods have brood pouches), and the brood hatches, you have REAL problems. There are only three courses of action in this situation; and I truly am not jesting about these responses. The first is to remove all the fish from the tank and wait the two or three months until you are certain that all the isopods have died from starvation. The second solution is to effectively nuke the tank. Remove all live rock and discard it as the isopods may hide in it, and as some of the isopods bury in the sand, you should also remove and discard the sand.
You may of course take the third option and do nothing. The most likely outcome in this situation will be that the isopods will kill your fish one by one. You may not even know they are attacking a fish until the fish is dead. Some years ago there were several large sharks that washed ashore in Florida. Examination showed that they were all killed by cirolanids that had burrowed into the shark and eaten their hearts.
Unfortunately, no easy solution. I would suggest setting up a hospital tank, and once this parasite latches on to a fish, then move that fish to the hospital immediately. You may luck out and only have one in there, but you will have to diligently watch all fish over the next several months and remove any that have the problem. In a bare bottom tank with PVC for decor, you may be able to remove the parasite if it drops off again while the fish is in hospital, if you're not up to picking it off.
Needless to say, you can't get any new fish until the situation is resolved. It may be wise to change stores where you purchase live rock and fish, since that is where the infection originated.
For a lot more information and various methods of dealing with this, type in "The Great Cirolanid Hunt" into the GOOGLE search engine, then click on the first link.
