it may just be the siamese fighter is being a tempermental little sh*t, as they quite often are , lol, even my goldfish have days where they mope around the plants, and some days they will not keep still.
is the fighter fish still eating and deficating (didnt want to be quite as blunt as saying shi*ting) as usual?
ideally it would have been best to quarantine the molly instead of treating the whole tank as the fish you mentioned are not as hardy as most coldwater specimins (where you can get away with dosing the whole tank). some fish will experiance side-effects to treatment, especially the less hardy varieties
chances are if you've been cleaning the filters in old tank water and only doing half at a time (as you should) then any trace of ammonia would be gone, and because there is no Nitrite reading it will most certaintly have already established a nitrifying bacteria colony.
common practice with any illness is to do a 50% water change and maybe up the weekly water changes from 15% to about 20%
is a lot to take in, lol.