One option is the Fluval M series heater, I currently run a 150 Watt version in a bigger tank with no issues, fully submersible and though not completely invisible in the tank it's at least smart looking.
Have a look, lowest wattage they have in this series is the 50 Watt version, it's made of borosilicate glass which is somewhat reflective so should blend in nicely.
http://www.fluvalaquatics.com/ca/freshwater/aquarium-heating/series/m-series/
Though this is rated for around 50L (15 US Gal) I've seen people use these in tanks of similar size to yours. However, you might be able to find other smarter looking heaters with a slightly lower wattage in the 25 Watt range which should also be enough to heat the tank.