It could have to do with the vegetables decomposing in the water. Possibly a fungus or something like that. Pics will definitely help.
However, you have a bit of an issue with your stocking. Otto's are social and like to be kept in groups of at least 3 fish, with more being better. They are also supposed to be in a minimum tank of 10 gallons. This is just something to contemplate, because an overstocked tank is an unpredictable tank.
><(((º> Aaron ><(((º>