confused.... temperature issues

So temp in tank at night did not drop at all. Couldn't figure out why my ac in the house is at 75 and my tank is at 80. Started shutting everything off to determine what the cause is. I have zeroed it down to my S2 running my 25w UV. Ran my S2 with the UV turned off and temp did not drop. Shut off S2 and it started to drop. S2 has been running for about 2 months. When I look at my apex I do not see the S2 using a lot of power. S2 was set to 30%. I also have an M4 for my main return set @ 30%. When I touch the pump (S2) it is not warm to the touch. I am very confused... anyone have in insight/love for the perplexed reef newbie? Thank you for the help!!!!!!