? on Stealth heaters

4 Stealth 250 watts in a 150 gallon rubbermaid sump that fills to about 5 inches from the top. Are the heaters going to sense warmth from eachother and shut off faster than they would if they were say in 4 seperate corners? I guess what I'm asking is how do these heaters work? Is the knob just a preset value and work like a potentiometer or is there actually a t-stat inside?

My heaters are pretty close to eachother in the sump and I'm just wondering if the proximity would be hindering the performance? Its cold in my pond and just want to make sure I'm getting all I can out of them before I go buy more or different ones.

Open to suggestions on a better/more efficient way to heat a 1.5K gallon pond in a RENTAL house.

Thanks in advance folks!

1.2 -1.5k gallon pond.

Temp was 82-84 now its down to mid to upper 70s.

Its a ray pond hence the reason for the heaters being in the sump. I moved them out but doesn't seem to be helping. Ill have to rig up some more gaurds then ill try repositioning again.

Was hoping to figure out how they work but I guess ill figure it out eventually.

Basement isn't that cold. I was thinking the issue may have been contact time as the pump is around 5kgph.