Tattered Fins

No matter what I do I can't seem to keep my LF bn's looking prettty. I have a 50 gallon tall (corner tank). 4 young bn's and one brown female lf is especially tattered. My LFS said use tetracycline but I don't think its done one positive thing after over a week. So I will be doing a 5o% water change tonight or tomorrow. I'm starting to feel its not an illness but maybe a vitamin deficiency. The two other lf's have some fin issues but not as bad as her. And I did have sunken belly issues with her in the past. I feed algae wafers daily. usually have cucumer or zuccinni in the tank. Not sure what to do anymore I just hate the look of her poor fins. Other than that they all seem totally fine. (except my short fin calico male did have weird redness in his fins and body but it went away, and when i treated with tetracycline my albino seemed to also get a little pink which I'm assuming was him being irritated by medication.) I do have a ton of gravel poop eating nematodes in the tank which i have read a lot about and am told they are good not bad. The tank is very cycled, lots of wood and some java ferns. All water parameters are 0 every time I check even nitrates. pH is about 7.4-7.6. Right now there is salt in the tank to see if it would help the healing process. but nothing seems to be changing.

If anyone can help please do!! I want my babies to be healthy and pretty again!!