I was thinking of having a rad cool the water before it goes back into the tank. There will be fish but I'm not worried about them. In fact they will be dead before the water temp starts to stress them out. The tank will just be for feeder fish for my bigger cichlid and giant gourami. ( don't have the gourami yet) and I assumed there would build up in the cpu block but it wouldn't be that bad. The pump I would be using has a flow rate of 900GPH so build in the block doesn't worry me a whole lot.