12-29-2010, 10:13 PM
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Not an expert by any means but it sounds a little like mouth rot, I would look up more on mouth rot and move the tetra to a hospital tank ASAP or start treating your tank with an antibiotic designed for mouth/fin rot. It can and probably will spread to other fish and can be difficult with angels in the tank. Constant water changes are very important if you wait to get meds in the tank. And if you do have a hospital tank move the sick fish in and preform large water changes on your normal tank.
Hope it helps, it's never fun to have a sick fish.