Internal parasites possibly. Is the poop segmented?
Praziquantel - for tape worms and some parasites
FlUbendazole(Kusuri Wormer Plus from the UK) for the rest of the worms and it also has a mild effect on Hexamita if dosed a couple of more times.
New Life Spectrum Hex Shiled food for internal parasites, e.g. Hexamita/spironucleus.
I'd get the Kusuri wormer plus and the Hex Shield. Follow the intstructions on both, can do it at the same time. If that doesn't sort out the poop. Do the prazi afterwards. Prazi is effective for worms that are normally caught in the wild so aquarium fish are less likely to have those but is also effective against some opportunistic parasites that the other meds can't sort out.
All are quite safe unless you have inverts in which case the Flubendazole can some.
If the poop is segmented, try prazi first.
Don't separate the fish, treat all.