1st - if you don't live in an area where the water would be 78- 80 degrees then you definitely should buy a heater.
2nd- I'd check the water quality.
3rd- Is the white spot really tiny? Are there more than just that one on the gill? If yes, it could be ich.
4th- If it's not ich, then it's most likely a bacterial infection. Usually fish with that just lie around in their tank.
5th- I'd definitely recommend this medicine Aquarium Pharmaceuticals PimaFix at PETCO
6th- If there are any other fish in the other divided parts, I'd treat them too. With diseases it's best to treat the whole tank to be safe.