It shouldn't be dangerous to get a 100W. You should test it for at least 24 hrs (IMO
) before using it with fish.
I have a 150W heater on my 20 gallon, it only really needs a 75W. The dial is set to a lower temp than the actual temp it keeps the tank at.
Current fish (includes all 3 running tanks): Polypterids, severums (of which I raised from eggs), gouramis, plecos (gold spot ones & a gurupa), loaches (zebras, yoyos, & a kubotai), macculocchi spotted silver dollars, an African butterfly fish, & Ctenopomas (a leopard & an ansorgii)