1800W is a helluva lot of heater for 240 gallons. In my 150 to keep a 20F rise I used 2 100W heaters. One in the filter tank, one in the main tank. Of course the tank was plastic, so it already had some insulation value.
Not to scare you, but don't buy cheap water heaters. I have had two Chinese-made aquarium water heaters stick with the thermostat on, roasting all of my son's fish. Hard to explain to him when he was 4 that that awful smell was his aquarium cooking his fish.
