I was suprised at his low res temps too. I have mine on a cement slab under the table in the dark with ambient room temps of 75-77 and my res is still at 71* right now. I only have a tiny 2 diaphram air pump for my air stones and it doesn't produce shit for heat in the air to the stones. I had another commercial air pump that basically blew hot air into the stones raising my res temps. Took me a friggin week to figure out why my res was getting so warm haha
I'm also using 4 pretty big air stones in there too..they make good bubbles...