Dont get the Hydor Mini, as Welsh said, it will overheat your tank because it's on 24/7 even when it's 100 degrees outside, the heater will be on.
You'll cook your fish in that tank. I made that mistake once and the temp went up to 87 degrees, thankfully my fish was ok. That was in a 1st galon too.
In my 1 galon I have the Thetra Submersible 50W and it keeps the temp at a constant 78 degrees. This heater doesn't have a dial but it does have a thermostat so it tuns on and off when needed.
A better option would be a heater with a dial that'll let you adjust the temp to a more suitable for the fish. I have both, the Visi-therm and the Hydor THEO. Both work very good but the Theo is smaller so it might fit inside your 1 galon tank.
Check fosterandsmithaquatics.com. that's where I got mine from.