Yes, and I do not recommend them at all. There's no temperature regulator, its either on (plugged in) or off (unplugged). Raised the temp in my 2.5 gallon by ten degrees! I just use the 25w Visitherm Stealthheaters in all my small tanks. Fits even in my 1g tank.
I would never recommend heating a 1 gallon with a Visi-therm but if it has worked for you I guess good luck. I would be afraid of frying my fish. I do love the Visi-Therm Stealthheaters though and that is all I use.
The little heaters in question are not too reliable. I had one and it had the temperature all over the place. I didn't have it long as it went more by the temperature of the air than the water for some reason and it had my tank too cool most of the time. No way of setting it as Boxermom mentioned.