Yeah the difficult part would be the water and oil initially mixing, well realistically speaking they'll then seperate into two completely different sections.
Kinda cool when that happens, mind I see and issue by using oil as a coolant; mainly the fact in order for it to work correctly you'd need to keep it constantly moving; here'in lies the issue with the water/oil combo.
You know oddly enough though, Amniotic Fluid won't short out electrical systems as it isn't conductive. It is breathable by the fish due to it being basically like a rich salt water... that said I doubt many have checked to see it's cooling abilities, plus getting ahold of 2-3 Gallons might prove a bit of a problem
That said why in gods name do you want fish in your computer anyway? I thought those who wanted bloody flashing lights inside were crazy enough. This is a bit like those old 70s platform shoes with Fish... sure they're grand until you realise you can't open then up and change the water and the fish die.
You fancy doing something productive, try to see if you can figure out a safe way of making LN2 cooling solutions. As personally I'd prefer a system that runs at sub-zero temperatures over something that has fish in it anyday.
