If you're talking about submerged PC's, I would strongly advise against it. Its not really that effective (or necessary for that matter), it voids the warranty on every single component, and they're difficult to maintain, cumbersome to relocate (its a friggin fishtank for crying out loud) and a nasty mess to work on if you have to swap a part or troubleshoot.
I guess the trade off, is,, it does look pretty cool, but its not something I would do with high dollar parts.