Where do you place your thermometers?

+1 on keeping heaters and thermometer away from each other. I keep my heaters in the sump and my temp probe is inside the DT, hidden in a corner.

 

IMO if water turnover from tank to sump is what's normally associated with reef tanks, then water temp. would be darn near the same at any point where a thermometer might be placed. I don't think it's possible for the tank to be a different temperature than the sump when water is so quickly moving from tank to sump and back again.

FWIW, my temp probe is between the baffles in my sump like xmetalfan. It's there b/c I can read the temp on the controller's display, while the probe remains out of site.

 

I keep mine in the overflow box , i see it on the fish room side of the tank.