Molly problem

I have a 10 gal tank with 4 platies and 1 molly - al but one is male. In
the past 3-5 days I have noticed the molly has a nearly transparent poop
trail and is not eating. It is a bit more isolationist than norm, but all
in all acts pretty normal. At feed time it will swim around a bit but not
eat. Nitrates are 20-30 but I am reducing them through water changes now.
Am, Nitrites - 0. pH 7.6, kH & gH both around 120, temp 78. I do salt
lightly (1 TBS every 2nd or 3rd 25% weekly water change.) We do feed
mostly flake food and there have been two 'batches' of fry in the last 3
weeks of which only 1 was seen/survived about 3 weeks ago and 2 the last
week. Not sure if the molly was the mom, though she has gotten smaller (fry
or not eating?). I doubt this is an external parasite/bact problem.

I would appreciate any suggestions about how to proceed.

Thanxx
Bill