if you have the room in between the rack and the tank. They make this stuff I think its call thermo-sheild. It looks like foil covered bubble rap. You could try that, or take the cheap way out stuff blankets around it. Not under or on top but three sides should do it.
And I agree with Tim, you have got to get the air temp around it up. Get you a little space heater and set a couple of feet away set it on a lower setting and see what it does to your temp. if it needs to go up then up it if it get to hot then lower it. The blankets and thermo sheild will help with heat loss once the air temp is up.
I wouln't incress the size of heat tape cause it would require the tape getting hotter granted for a shorter period but still. if you have it hooked to a thermostat then it going to try and get the temp where it should be and if that means getting the heat tape really hot then thats what it does. And then you have go a burned snake.
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