Oh my, i had a hampster once and that little sod used to chew his way though everything, if i left him inside my case for 10mins hed probably eat half my graphics card :-/
Having thought it through i've decided that the fishtank computer isn't possible
the only way to avoid blowing everything up would be to use pure de-ionised water and its not going to stay that way for long with fish and fish food in there !
maybe i could devide it in two with a thin sheet of glass and fill the computer half with clear oil or something so it look like the whole thing is the tank.. i'll have to think about it.
Or maybe setup some watercooling then set all the components in clear resin or something..