I use two 50w Jager heaters on my 24g.
Lots of folks swear by the Visi-Therm Stealths as well. I do not own one of those personally though. I have at least 12 Jagers and have never once had one fail on me.
I'm also a big proponent of using two half sized heaters instead of one large one. It helps prevent one "off" failure from dropping the tank temperature too much and one "on" failure from cooking it before I notice. Even though I trust my Jagers, I don't completely trust my tanks to any heater.
Size about 5 watts per gallon give or take. Don't go too far over. Too big of a heater can cause excessive heating (which may be your issue...
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