I need help with my coldwater fish?

me and my boyfriend have brought a fish tank that hold 35 gallons of water and today we brought a albino cory and 6 minnows the man in the shot had them in the coldwater side of the shop and sed it was fine they didnt need a heater. the minnows are doing great but the cory is not moving alot and swims on his side? does any1 knw wot it could be? do i need to get more as i have read they do better in groups?