Stainless Steel is not used in pressure vessels because it's not strong enough. It's corrosion resistance is also over-stated.

SIGFever, as I pointed out in another thread, two conditions have to be met for galvanic corrosion to occur:

1. Two or more dissimilar metals must be submerged in the same electrolytic fluid and,

2. The metals must be electrically connected.

The tank would be at earth-ground potential, obviously. I haven't built a water cooling system yet but I don't see why the copper blocks would be connected to anything. They're mounted to non-conductive chip housings, right? I notice there is usually a screwed-down clamp over the block. If the screws contact the case or the pcb ground plane anywhere, the clamp should be insulated from the block.

You CAN have aluminum and copper in the same loop as long as they are not electrically connected to each other.