Here is a neat little calculator for figuring absolute minimum wattage for fish tanks. Select your dimensions, room temp, desired water temp and also glass or acrylic (yes acrylic makes a huge difference) and presto.
http://boonedocks.net/fishtank/ftweb.php
I've got a 300 gallon acrylic tank with a highly insulated 200 gallon sump that is controlled perfectly with a single 250 watt heater. But I'm only raising the water from 76° F to 78° F right now. This winter the house will be 68° so I'll probably add a second heater for insurance.
http://boonedocks.net/fishtank/ftweb.php
I've got a 300 gallon acrylic tank with a highly insulated 200 gallon sump that is controlled perfectly with a single 250 watt heater. But I'm only raising the water from 76° F to 78° F right now. This winter the house will be 68° so I'll probably add a second heater for insurance.
