I have an Aqueon 250w running on a Reef Octopus Digital Controller (I don't trust those cheap thermostats to not stick eventually) in the 30g sumo under my 80g DT. I'm going to replace the heater with a 300w Titanium, just because, even though the Aqueon 250w kept the tank within 0.5*F at all times, even before the controller. I agree a 500w heater on a 90g is overkill and possibly an instant oven, if anything happened to go wrong, God forbid. 300w would be plenty on your 90g, or 2 150w's (I think that's what was said) as previously suggested.