Cold Water Fish
I noticed that you mentioned coldwater aquarium. With no heater there are still alot of options for fish you acn have. White cloud mountain minnows prefer cooler water, down in the 60s and low 70s, as do hillstream loaches. Several variety of freshwater shrimp prefer cooler water as well. My loach tank linked to in my signature is a "cold"water tank as it has no heater. It gets down to about 68 in the winter, and up to 80 in the summer. Most of the fish prefer the winter temps though.