My temp starts at around 78 degrees in the AM and then it gets to around 79 to 80 degrees by evening (lighting and such). Like phases states stability is the most important factor, the ocean, being one huge aquarium has untold millions of gallons of water and the variance of temperature in any given spot is pretty consistent. Try to keep the temps at a stable setting by watching tank placement and proper ventilation and cooling if necessary of water circulation and lighting.