This is a hard one. I've been thinking about it all day.
The problem with Tubifex is that they are happy living in polluted water. I don't know what the Ganges river is like. I've been told that the Asian fish stores all sell them. Apparently they know how to clean them up without killing them.
You need to talk to a Discus keeper who is by the Ganges if there is one. Sorry I'm not of more help.