I dont think a pc inside the aquarium itself is going to work. fish cant stand big temperature variances very well (at least, as far as I know they dont. ) and if the pc would be used for quite a wile, the water would warm up for sure!
YOu can waterproof most of the bits tough with plastic spray. (its a spray that puts a plasic coat on PCB's and Components. I use it for electronic speed controllers in my rc cars, so they are a bit better protected against water. this however wopuld still leave a problem of the CPU socket, and all otehr connections on the motherboard.