Will putting vaseline on a pimple help?

What treatment can I use for my fish tank?

Recently one of my new guppies has developed what I believe is fin rot from another guppy nipping it. I think it may be developing onto ick as I see another fish with whiteish stuff on its skin as well as this one with the fin rot also has whiteish stuff ...  moreRecently one of my new guppies has developed what I believe is fin rot from another guppy nipping it. I think it may be developing onto ick as I see another fish with whiteish stuff on its skin as well as this one with the fin rot also has whiteish stuff on its skin. So I wanted to know if anyone knew of any medication I could use to try and cure them. It would be preferable that I can find it at a Petco or Petsmart. It also has to not harm any live plants or nerite snails because I have those in the fish tank as well. Thank you for your time!  less