well if you set your temperature @ 27C then its should be the fix temp for your tank forever. you have to maintain a constant temperature for your fish since a small drop or increase of the temperature can stress your fish. fish are prone to white spots when they are stressed so better take good care of them , check your water parameters if its right. check your PH levels, amonia and nitrates should be low or zero. if you fish got the whitespot already then you can cure them with medicine like (tetra) contaspot. follow the instructions carefully and you should be succesful.