I have a dwarf papyrus in a galvanized container that I keep full of water. Problem is, it's standing water and I discovered mosquito larvae squirming around in it yesterday. The papyrus dries out too quickly and turns brown if I don't keep a two or three inch layer if water in it, but I don't want to encourage mosquitos.

Is there anything I can treat the water with to keep mosquitos from laying eggs in the papyrus? I'd get a mosquito fish, but I don't think I have a deep enough container for the fish to survive.