Yeah, sounds like internal parasites. Usually what happens is the fish stops eating, then becomes lethargic--poop is abnormal during this period. I highly recommend a medication with praziquantel in it. I have had some success with Jungle's Parasite Clear tablets. After you begin treatment, your fish should start to pass the worms--lots of poop! Generally once they start eating again, they're fine.
Fortunately fish generally respond very well to de-worming, and praziquantel is a lot less harsh than antibiotics. I recommend picking some up ASAP.