[Delphinus]

How much turnover do you have in your sump compared to tank volume? I've always heard the general rule of thumb that you need around 5 to 7 times volume turnover through the sump to keep heaters down there. I'm sure you have this or knew it already but it bears asking.

What's the ambient temperature in your house? All things being equal your tank should equalize to that after some time and you would only need to heat your tank the incremental difference. Ie., house at 21c the heaters need only raise it 5c to get to 26c (sorry I don't think in terms of F for temperature. The very first dive I ever did was at the Great Barrier Reef, the water temp there was 26c, and that's what stuck in my head mentally ever since). If you run your house at say a chillier 18c the heaters need only bump it 8c to get to 26c. That's not a lot.

So I'm thinking that either the heaters aren't outputting what you think, or there is loss of heat to somewhere (the floor? is the sump sitting directly on concrete or is there foam under the glass?)

If all 4 heaters were on that should be the full 1200w which seems ample to me. By contrast, my 280g, I use one 1000W titanium heater on a Ranco controller, the RO/DI was a frigid 8c filling up, and the heater brought it to 26c within 24 hours (not sure exactly how many hours it took, I just checked the next day and it was 26c). It's been steady ever since. So I think your 1200W should be doing the trick, there is something more going on here that we haven't put our finger on methinks.