It has also been noted by a few people that Stealth heaters are not particularly effective. I have 3 in my 200gl tub. The plastic coating on them is a bit of an insulator, making them slightly less effective than an aluminum or glass heater. The element may be 250W but that doesn't mean they put 250W continuous into the water. Just a thought.
That said I also have a 200gl with 2 250W stealth heaters on it, and it stays at 80, no problem.