I have 7 250 watt stealths in my pond. Drafty cold basement. Pond is insulated with 1.5 in pink styro on bottom all walls and covered with it at night. Pond is roughly 1K gallons and I can hit 90 degrees.
DO NOT aim to be able to keep at 80 but aim to be able to reach 90 if you need to.
Stealths are cheap from Pet Mountain and if you buy a grip shipping is free. I'de get 5 in your application. Start with 3 and that will probobly get you where you need to be but if you run into problems and need to hit 90 then you'll probobly need those other 2.
DO NOT aim to be able to keep at 80 but aim to be able to reach 90 if you need to.
Stealths are cheap from Pet Mountain and if you buy a grip shipping is free. I'de get 5 in your application. Start with 3 and that will probobly get you where you need to be but if you run into problems and need to hit 90 then you'll probobly need those other 2.