I have a Hydor heater, too and I've had good luck with it. I only plug it in when the room temp is below 75 degrees or cooler. I've buried it under the gravel right below the intake for the filter. The first time I plugged it in, Claude played under there as though we'd just installed a new hot tub!
I'm in FL, so it doesn't get plugged in too often even in the winter. Usually only on the coldest nights, since we use our furnace very sparingly. I keep an eagle eye on the thermometer to be sure the temp stays pretty stable. I don't mind if it drops a bit at night and is warmer in the day, since that would mimic nature - as long as it doesn't drop out of the "green" that is! Can't give the fishie a bankie, afterall!
At any rate, Jessie, I'm sending good thoughts to you and Sparticus. Please post and let us know how he's doing - if you notice any other details or symptoms, pass them along, perhaps we'll be able to help.