Is your tank covered and what type of light do you run. I have an 80 gallon tank that I keep at 80°F no problem on a 200watt Fluval M series. I actually had to dail the heater down to 76 because of my light. Running a Finnex 24/7 SE. The light plus glass tops trap the radient heat; have you considered location of your tank and circulation within. Im suprised that at 55 gal. That 200watt isn't getting it done. I know there are fixed heaters out there like Tetra, and there is a +/-. My brother bought one and the heater is set at 78°F with error of 2°+/-.
May be defective. Try another and be sure to have lots of water movement.
If you email aquarium coop directly they may be able to answer. I don't know if they will get your question via Google.
Maybe 2 heaters could work
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