I have a 50 watt adjustable for my 10 gallon and it keeps it a toasty 78 for my white clouds, and I have a 100 on the way for my other 10 gallon.. so I'd say higher than 100 for anything more than 10 gallons
it should say the watts right on the heater.. it's the only reason I know the one in my 10 gallon currently is a 50 which I am surprised heats the tank as well as it does.
Thanks :') My other heaters are made by Eheim, and I just found their guide online, and it says 100 watts for 26-40 gallons. I think I might not buy the 200 watts one, I just thought maybe I could, because it's so cheap. Thank you :)