If one of the tanks will have 25 degrees, I would think that the heater will run permanently, untill the whole tent becomes 25 degrees. Because of the permanent circulation of the water, the 25 degrees water will cool down during the passage through the growbeds, and has to be heated to 25 degrees again in the tank. This will continue till the whole system reaches 25 degrees, or, and this is probably more realistic,
the heater will run permanently, and there will be a balance between the heat input and the heat loss, resulting in a permanent temperature somewhere between 20 and 25 for the whole system. My guess would be that this would mean a serious increase of the amount of energy used, especially during these cold days