That's a lot of wattage lol. I was amazed at how well my 240g holds heat being acrylic with the big 3d Bg I think all that foam helps. Because just last weekend I had a heater fail, noticed my normal 80 temp was all the way up to 84. I had two 300w heaters in it. I took the bad one out and amazingly it holds 80f with just the single Jager 300w heater now, and that heater isn't even really having to work hard, it's only on maybe 50% of the time. I can't figure out how it is doing it to be honest because the tank is in my basement here in Michigan and it gets down to 62f room temp at night down there.