Fish suffering from internal parasites can be identified by the fecal discharge, which is usually white in color and appears as long transparent trailing string. Some might even show more severe symptoms with a sunken or bloated belly and usually the fish often become lethargic with huge drop in appetite. Sometimes the most severe cases even have the poop containing tiny whitish colored worms that are still alive.
Although there was not any conclusive evidence to pinpoint the exact source that could cause this fish internal parasites, I always believe it is all related to the live food that you feed to your fish. Most of these contaminated live foods are usually tubifex or bloodworms that were obtained from contaminated waters at drains or sludge area whereby there were presence of these parasites. Therefore it is advisable that you get your live foods from reliable pet stores and check carefully with the salesperson the source of supply. That is why some aquarist even go up to the extent of ensuring continuous supply of live foods by cultivating their own microworms.
Thankfully despite all the complications, there are couples of choice of treatments to deal with the internal parasites. I was told that malachite green can be an effective form of treatment but somehow I feel that this is not sufficient. The best way to reach towards the source of problem is to supply your affected fish with medicated food that contains antibiotics. I wouldn’t recommend any particular brand here but basically all the commercial brands produced by top manufacturers should be reliable to come up with the right formulation. If everything fails, hopefully there is still time left for you to bring your pet to the veterinarian for treatment.
Other Aquarium Fish Disease: Lice and Anchor Worm External Parasite