The newest Sera aquarium heaters are specifically designed for low aquariums. The 25 Watt version is suitable for aquariums from 0-25 liters.
The Aquael Comfort Zone Fix are designed for maintaining aquarium water at a constant temperature of 25 degrees Celcius in the Aquarium. For aquariums of 10-25 liters.
With the Eheim thermocontrol-e heating element your fish get the right temperature even more precisely - in every aquarium. For aquariums up to 25 liter.