Trick is to get the water to temp so the heaters turn just off then pinch em up, I would set the temp 3 degrees or so higher than your goal you should not have to worry about it dropping too low. Do you have a sump? If you do put the heaters in there It will keep it more consistent. I run my water hot at like 85 and I'm working with alot of water, but, I ran 2 rena 300 in my 135 and I had no problems at all. Get a good thermometer and keep checking it. If it dips at night 2 or 3 degrees really won't be too bad because the temp doesn't fluctuate rapidly, it's gradual enough for the fish to deal. Do you have glass tops on that tank? They do a better job insulating and will keep it more consistent as well. Pm for more help if needed.