How much temperature fluctuation is normal for a 29-gal tank without stressing or harming the fish? I'm talking overnight temp changes within the course of a few hours. My house gets cool at night and sometimes I worry that the fish look sluggish if the tank temp has dropped 3 or 4 degrees (F). Of course it doesn't help that the sun rises later now so I see them when they are completely asleep. BTW I do have a heater but I guess it doesn't keep up all the time.