The newest Sera aquarium heaters are specifically designed for low aquariums. The 50 Watt version is suitable for aquariums from 25-50 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 25-50 liters.
With the Eheim thermocontrol-e heating element your fish get the right temperature even more precisely - in every aquarium. For aquariums from 25-60 liter.