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[wokka]

How big are the tubs? whats the room temperature? Is there a back on the rack? Is there a front on the rack? How much ventilation do you have?
Electricity via, in your case, the heat cord is capable of heating a certain amount of mass being wood ,plastic, air etc by acertain temperature differential.
A certaian amount of electric power (wattage) can lift a larger mass a small temperature differential of a smaller mass a larger tempature differential.
My observation is that about 50 watts of heat will maintain a cubic meter of air about 10degrees above the room temperature if there is no draft. eg to keep a cage 1m x.5m x.5m at 30 degrees in a 20 degree room the 50 watt heater would be on about 25% of the time.
My guess is you have too much volume, too much draft/ventilation and too little wattage.