Aquarium heater/separate temp probe = bad with corn mash

[nerdybrewer]

I have a very thick corn mash with the grains, also oats & wheat in there.
I use a titanium aquarium heater with a separate temperature probe and a controller that shuts off the heater when a set temperature is reached, turns back on when it cools.
Unfortunately the thing sits there and cooks the corn that's nearby and the temperature probe never senses an heat even if it's just a few inches away.
I have strained out a lot of the floating grain, skimmed off the burned stuff and tossed that first.
I'm hoping that with some of that grain out it would maybe allow me to put the heater back in and warm the mash up, it's sitting at about 55F right now.
I have a heating pad strapped to the outside of the container right now but it's not doing much to warm anything up.

Looking for ideas? This isn't going well and I've got over 50 gallons of mash sitting there cold.