why are you worried about cold water, thats what we all strive for unless its clones or
seeds. Obviulsy not freezing water but your not even close to that. wait tell you have cycled your
lights and you'll get some heat. The big question is will you need air conditioning or a water chiller once the lights are running.
I just walked into my green room and it was a bit warm but not enough to activate my emergency program. went to the graph and saw the system was running my air conditioning re boot and de ice policy. the policy is not allowing for enough time for a full melt of the coils. the humidity sensor failed to signal the relay to activate it so the humidity got up to about 53% relative at 85 f. Little jungle love going on there.
I excuted a re boot and went through the days grow jouranl and found the error.
So a cheap way to heat the water for a few days for clones or seeds if you must is just to paint a floresent bulb black and stick the glass part of it in the water. hook it up to your light timer if you want or just lift it a bit out in aday if it gets warmer than you want and you can leave it on all the time you need to.
Fish tank heaters in plastic tanks is a disaster waiting to happen every time. I cant count how many times i drained a tank and forgot the heat was on it.