yeah, go with a 50watt, FULLY submersible. Expect to pay at least $25, but its worth it. Cheap heaters tend to not hold a stable temp, and often the thermostat breaks and it'll either freeze or cook your fish.
Rena Cal, Ebo-jager, Visi-therm, Titanium...these are the heaters you can trust.
Do NOT buy a Whisper or Tetra-tech heater.
Former advisor and planted tank geek...life's moved on though.