Otocinclus

Otocinclus catfish are the most peaceful addition to any fish tank. In fact, these small catfish which grow up to about 5cm are widely regarded as THE best algae eater for any aquarium. They can live in nano aquariums too because of there small size. They are extremely peaceful towards other fish and will  not even eat your plants. They are the perfect addition to a planted aquarium. 

If your aquarium does not have algae they can be fed boiled vegetables like cucumbers and carrots, or specialised foods such as algae wafers.