A fish friend of mine keeps his African and CA cichlids in outdoor plastic pools during warm months. He lives near Philladelphia and he said that in spring and fall time, the temperature occasionally will drop to 55F at night, but his cichlids are thriving and breeding in outdoor pools with no suplemental heating. He is more concern with heat waves because when the water temperature gets too high, it can be depleted of oxygen and he has to add well water to the pools to cool down.