The 8000-gallon outdoor aquarium is heated with standard 500 and 1000-watt titanium heaters. The thermostatically controlled heaters reside in the 1000-gallon sump, which circulates continuously through the filters, and back into the aquarium. The total of 2,000 watts is sufficient to maintain a 77-78 degree temperature year around in well-insulated aquarium.
Incoming city tap water during the winter is in the range of 37-43 degrees Fahrenheit and must be heated. In the sump wall, I have mounted four 5500-watt common water tank heaters threaded into 1-1/2 bulkhead fittings. During water replacement at the rate of 300 gallons per hour, I temporarily turn on a total of 22,000 watts, which are on time clocks.
My greenhouse aquarium has a glass roof four feet above the water surface. The aquarium is entirely covered with glass panels as well. In the day time radiant heat will warm the water somewhat even on cloudy days. When the sun is out the area above the aquarium and under the glass roof can reach 90 degrees or more. Photos below.