How big is the tank because a 50watt heater only good for a 10 gallon tank.

Rule of thumb 2-5 watts per gallon.

Some advice get a heater that has a heater guard to avoid your turtle from getting burned.

I’ll highly recommend a Fluval E series heater

https://www.amazon.com/dp/B001VMSK0I/ref=cm_sw_r_cp_api_glt_fabc_A9F652PHC6FBEK2NTM5F?psc=1

They come in 100, 200 and 300watt models.

E than wattage is how the naming goes