Black Widow/skirt Tetra Illness?

[stacelou]

Hi I have a 130 litre tank with happy tank mates. Had all fishes for at least 6 months together including tiger barbs pleco kissing gourami and a few other tetra, I regularly do water changes once every 1/2 weeks but haven't checked the water quality in a while so do not know the ph levels or nitrate levels I do know Im naughty and should change water more frequently and should know my water quality, all other fish are happy feeding well and have no problems but one of my black widow tetras has a tear in its fin/body. The injured fish seems normal swims fine eats normal but the tear looks painful. It started off as a hole in the anal fin but now looks like the hole has been tore and has "ripped" the body also. I haven't added anything new to my tank recently, most recent thing added was 2 plants added a month or so ago.

 

[fluttermoth]

Can you give us the exact number of fish, of each species, please?