How to help guppy with fin rot in a community tank

[GuppygirlLol]

Hello,
I have a cycled 20 gallon tank with 4 guppies,
5 galaxy rasboras, and 5 panda corys. also one mystery snail. One of my guppies was bought recently from the LFS and may have been nipped but another guppy in my tank. Despite my water seeming good, (0nitrires, 0 ammonia, 7ph, 5 nitrates) She has developed fin rot with uneven decay with redness at the edges. I had to leave for work but I would appreciate any advice. Should I quarantine her from the others? Or should I treat the whole tank? I have lots of plants etc and I worry that the other species of fish may be more sensitive to medicines and salt. Can panda cories handle dosing of salt? Do you think it’s bacterial or fungal? . and with it being noticeable and worsening that past 2 days, is my whole tank already contaminated? Thank you.

 

[Colin_T]

can we get some pictures of the fish?

all freshwater fish can tolerate low levels of salt (1-2 heaped tablespoons of salt per 20 litres of water).

 

[GuppygirlLol]

can we get some pictures of the fish?

all freshwater fish can tolerate low levels of salt (1-2 heaped tablespoons of salt per 20 litres of water).

 

[Colin_T]

It looks like someone took a couple of bites out of the tail.

The red is blood from the wound.

There doesn't appear to be any fungus on the tail. That normally shows up as white fluffy stuff.

Do a big water change and gravel clean the substrate, then add some salt.