MH REALLY add that much heat :(

[fkshiu]

Originally Posted by banditpowdercoat

Ohh, if they use water for cooling, t hat I can see, but my 2 externals don't use water for cooling. The mag, yes, its submersed. only 43w though, wouldnt have thought that would be enough, but maybe huh.

Water still forms a jacket around the impellers of the external pumps thereby transferring heat. Not as much as an internal pump, but there is still heat transfer.

Mags are infamous for being heat generators.

[pterfloth]

I run 3x400w on my 200 gal tank. I use two small fans blowing across the top of the tank for cooling. When the lights are off, the tank cools quickly overnight, so much so that the heater kicks in to maintain temp.

It sure sounds like your pumps.

[hillegom]

I think it would also help if you get rid of the heat in the cabinet. For sure that heat will warm your sump and then to the DT

[Aquattro]

For the question, do the rocks retain heat and add it back to the water after the lights go out? No. Once the lights go out, temp should drop again.
For pumps, my MAG 12 is the only thing I use to heat my 20g makeup water tank, it keeps the water at 81. Pretty much any pump will transfer heat, some more than others, submersibles transferring the most.
Fans pointed across the surface will help a lot, but will evaporate a lot of water. This is called evaporative cooling and just for that reason
The most stable method to keep temp set is a chiller. That being said, if you only swing a few degrees each day, it's probably not a concern.