RickS-
I think you mean watts, not KW. 3000 KW would boil 900 gallons in no time. 2 Kw is what I used in my 900 gallon basement system. Heat needs depend a lot on ambient temperature and how well insultated & covered your tanks are.

If you have access to lots of cheap wood, a snorkel stove can make sense. At 10 cents a KW hour and 2 KW heater will cost you about $5 a day or $150 a month. A winter will repay you handsomely assuming you can control the beast. I personally would use it to heat another tank used to supply hot water as needed to keep other tanks warm, rather than put it directly in the fish tank.