How to keep fish in a too small container...ie...1.5 Gallons.
1. Large water changes every other day. (75%+)
2. Include small heater noted in previous post and NEVER turn down your thermostat for any reason thereafter. And place aquarium away from windows, outside walls, and heating vent/radiator.
3. Do NOT overfeed. Feed maybe half of the recommended amount on the food container.
4. Be prepared for a lot of work OR be prepard to lose fish. Small tanks are NOT for beginners.
It MAY work out for you, but more likely it will go badly. As everyone else here has been hinting at...1.5 Gallons is TOO SMALL.