Yes, a fish tank can function as a reservoir.
Yes, a fish tank can function as a reservoir.
Technically... if you limited yourself to a 5-10 gallon tank, you could fit a Beta there. A single fish won't generate much waste, but adding a bit more filtration would help prevent your small creature from getting sucked up. Regular tank cleanings would still be necessary, replacing most of the water with fresh water. Turtles create a lot of waste, so I wouldn't suggest an amphibian for this setup. Goldfish also produce a significant amount of waste, making them ideal for jump-starting a new tank, but Beta fish don't require the same level of bacterial activity to manage nitrites and nitrates. That's why you often see them in non-filtered environments.
I evaluate multiple reservoir choices well before settling on the 'fish tank' option. It comes with a lot of mess. What maintenance will be required for both the fish tank and the water cooling system? Is this a continuous solution? If not, you'll have to account for fluctuations in water temperature during operation and when it's not running. This idea is more of a passing trend than a practical solution.
I would arrange the setup, test the empty tank, and observe the temperature range of the water. Then identify a fish species suited to those conditions. You could install a thermostat-controlled heater to maintain suitable warmth, along with a bubbler and filtration system to prevent overheating. As the cooling system begins adding heat, the extra heater's workload will decrease. I’m also thinking about creating a separation within the tank so that waste or algae from the heat sinks don’t reach the water, possibly using chemicals in the back section to manage algae growth. Regardless, the radiator should be fully sealed to keep the tank water distinct from the cooling fluids (still deciding on the fluid type).
A big reservoir will extend your waiting period before the loop stabilizes. The bigger it is, the more time you gain. You should definitely look into 4ryan6's sub-ambient loop—it relies on a very large reservoir and uses frozen water bottles to lower the temperature. You might find some of these concepts useful; check it out. Is this your first water cooling setup? Have you tried the WC sticky?