I run a Hydor 200W external on my 25 litre nano tank, because I wanted to keep all equipment out of the tank as far as possible.
That wattage is obviously far too much for the size of tank, so I drive the heater from an STC-1000 temperature controller that I’ve mounted in a plastic box. If either the controller or the heater should fail in the “on” state, the other should cut off.
Whatever heater you use, I’d recommend doubling up with a temperature controller so that you’re protected in the event of the heater sticking on - I understand that’s the commonest failure mode.
Hydor 200W external on my 60p tank. These were unavailable in Australia for a while because I think they have redesigned them to avoid the failures they used to have. Although currently it is unplugged since it is no longer winter.