Heater Recommendation Please

[Cliff]

I currently have two very old 300W heaters in my set-up. It’s a 90 gallon with a 55 gallon sump. I’ve had these two heaters in there for about 1 to 2 months now. During the past 3 days, I’ve noticed my heaters are starting to struggle to maintain a constant temp. It used to be at a pretty constant 79 F and now it’s ranging between 76.5 to 79 F. There has been no changes in the room temperature during the 2 months.

I want to up-grade my heaters with one (if possible) or two other new heaters

I’m looking for recommendations