That heater will easily heat that much water with out a problem, Stop listening to those that are saying you need more heaters and actually look at the faulty heater or you do not have it adjusted correctly, Crank the thing up to max and if it heats super high great it works and you need to adjust it, turn the thing all the way down and it stays the same then you know its a bad heater. With mine that I had the light stayed on all the time no matter what I did, all the way up all the way down and it always felt a little warm to the touch but was overall just not heating like it should. Unpluged it for a couple days, adjusted everything, even wrote stupid customer service and told them all my testing and that I had adjusted it back and forth every which way and all they could write back was try adjusting it some more. They never once took an opportunty to have me send it to them and get it replaced or anything, and I just purchased it, never used before. I have 4 of these heaters on my 600 gallon tank and they are so so in my opinion, all have a little moisture in the glass since putting them in new, but at this point they are about 6 months old and still running alright so we will see how long they last and if I care to buy more again. I run my 120 with sump off a 250 watt Aqueon Pro heater. Lifetime Limited Warranty I really wish they made a 300 watt model or larger I would buy those exclusivly. But so far my 250 watt unit has been great, always kept my tank warm and like that I can tell that its working and on all the time.