yes, you'll need a heater. having the temp bounce around like that is not good at all for the fish. a good heater is the marineland stealth pro. it'll keep it perfect.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry