26.8 litres is approx. 6 gallons. Ask your LFS for a heater suitable for that size tank. If it fits in the tank, a heater suitable for a 5 gallon might work or one suitable for a 10 gallon.
edit: the hardware section would be a better place to ask
I've just got a new tank!
I've set up the decor and added water etc. and I've just put loads of food into it to help getting it cycled. How quick will the flake food i've put in breakdown and disappear?