The heat from a pump might affect a smaller tank, but my 300 gallon tank is more effected by room temp. Before replacing the Mag 12 with the DCT-4000 I was running a 175 watt heater
as the over night house temps were dropping to 66 or 67 degrees causing the tank temp to drop to 74 and below.
On the other hand, during the summer if I leave for several days, the house temp can rise 80 degrees or above without the air conditioning running.
I would leave the air set at 79 or 80 to keep the tank below 85 degrees. Now with a cooler pump, maybe I could set the air conditioning at 81 degrees.
My estimate is that the air conditioning runs at 5000 watts. So running that less, certainly makes up for running a 175 watt heater for a few extra days.
Pump Update - Still running after 5 days.
Pump Update - Still running at 15 days.
Did replace a Mag 7 with DCT-4000 that was running as return pump. I'm running that pump at 40% ( 18 watts ) That's 21 watts less than Mag 7.
The tank does now run cooler without the 2 Mag pumps. The house temps have increased, yet the tank hasn't.
I am running a 175 watt heater that before the switch to DCT pumps wouldn't be required in March.
No two ways about it, the DCT pumps run cooler in the tank. All in all that's a good thing. Yes, I will have to heat the tank during the coldest months.
But, having the tank run MAYBE two degrees cooler in the summer is a great trade off. The tank is in the hottest room in the house, and I'm also in that room. I like cooler.
Second, with a cooler running tank, I don't have to worry so much about the tank overheating if I go away for a week in the July heat.
Leaving the house AC at a higher temp more than makes up for running a 175 watt heater in the winter. The AC runs at about 5000 watts.
Only downside so far. Not that the Mag pumps were loud, but the DCT pumps are very quiet. So quiet I can hear how gosh darn loud my protein skimmer is.
If one of skimmer pump fails again, I will replace the skimmer. I've seen several models using the DCT-DCS pumps, and am likely to go in that direction.