What do you mean by glass fish? Glass/ghost catfish? Or indian glassfish?
In my experience, I have 2 ghost catfish in my 10 gallon tank (I don't recommend anyone do this, they were bought before I found Fishlore, now I'm trying to rehome them) and they are a highly gregarious fish. They should be kept in shoals of AT LEAST 6, a single fish will likely die without the rest of his shoal. I've never heard of aggression amongst themselves.
Indian glassfish are brackish water fish and need salt in their water to survive.