New to Knives 8/2/04 I'm new on the knife fish thing so you might need to help me. I don't have any current problems but,  I want to know if three glass knifefish  in a 55 gallon tank is ok. < These fish have weak electronic organs and when they are kept together they seem to always be bickering  amongst themselves. They don't do any real harm to each other so if you have plenty of areas to hide then they  should get along fine.> Males grow about 18" and females 11". A fish manual says 1" per gallon that would be 54"  with three MALES. Here's the question. How do you tell male from female when they are the same size. < The books all say that the males get to be the larger fish. They are somewhat transparent so look for  differences in the body itself. You may not see any until they get close to adult.-Chuck> They are about 4".((I have other fish in there already.))  Please Help If Possible. Thanks.