What's wrong with my tetra?

do you have an aipump? how high are your nitrates, is your ph at 7.0 or lower, if it is not, all of your teras may be affected buy this issue, they can tolerate higher PH but if there was a sudden change, or if you did not carfully aclimate them before adding them to your tank, there can be issues, tetras are very sensative to water quality. if she has a wound, she will never recover in poor water condition. you can add melafix, but like i said, check your water parameters, it is very important and should be done every few weeks. minimum, more if you are having issues.