There could be several issues. Maybe your thermometer is off. What are you using? Have you tried other thermometers to verify?
What's your room temp? Your tank will be at least that much in temp. How large of water volume is the tank? Do you have good circulation inside the tank especially around the heater? What brand of heater?
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It's an aqua top it was only 19.99. I have my tank in the basement mid 70s down there. I have the heater next to my filter so it flows through tank. 75 gallon tank. Only other heater I had was an odyssey one amd it burnt out In a few weeks
Sounds like your investing in very cheap heaters. Its the one main physical piece of equipment you should never cheap out on. You do get what you pay for. Cheap heaters can spell disaster. Either by not working or by staying stuck on and over heating your tank. I'd pick a better quality heater. Make sure your thermometer is working correctly to.
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I use both jäger and fluval E-series and both work great. I really like the fluval as it has a digital display and the colour changes with the temp. It also comes with built in gaurd to prevent burns and breaking
Cheap heaters with a good quality controller is another option. I don't think you'll ever get the <1*C accuracy you get with a controller out of an in-built thermostat, even a Jager.