The biggest problem will be temperature control. You are putting in a certain amount of heat that you cannot change. You will need some system for either adding or removing heat from the system if it gets outside the range desired by the fish. What temp do the fish prefer?
70-80 is a good range for most fish....plus or minus 5 F would be no big worry. Is the water being directly used through the puter and in the tank? If so....Copper may be a problem.
Best would be a heat exchanger in a plastic bag or such so the fish water is not interacting with copper + aluminum. Likely-the limited interaction of a copper CPU heatblock would not be too bad-but it being hot MIGHT accelerate some copper getting into the water.
IF you are doing this in a LARGE tank....50+ gal,letting it supplement a regular thermostatic heater is great stuff-and you can set it up easy so the Computer water and fish water do not mix-but the temps equalize.
Even a long section of submerged plastic hose would give some heat transfer at no risk to fish. Of course you also don't risk sucking a small fish up and frying him on your CPU.