I got some anti parasitic medication today since I cannot get API General Cure at where I am nor on Amazon. The direction is to treat the water column for 3 days. I'll see if they get better.
3 day treatment will take care of any flatworms due to active ingredient praziquantel. However, if it is a protozoan infection, it is the metronidazole that is needed for that. General Cure does contain both active ingredients, but its misleading for users ( that 3 days treatment is all that is needed) because protozoan infections should be treated for 10 days. Also, metronidazole breaks down quickly in the aquarium-- so, its better to feed this medication in food with a binder like Seachem Focus or a food grade gelatin like Knox.
Unfortunately, one of them died from swim bladder disorder, with bloated stomach and loss of appetite. Will this be related to the other deaths or symptoms (sunken belly but actively eating ones)?
Internal parasites can be consistent with both the swim bladder infection and the sunken in belly of actively eating ones.
As a parasite infection progresses, a fish can either get on obstruction due to worms in gut, leading to secondary bacterial infection or the swollen belly/swim bladder issues can be a sign of internal organs shutting down from internal bacterial infection caused by parasite. Parasites are the underlying cause but it is infection or obstruction and infection that lead to death.
A lot of times the first indication one has of fish with parasites is a sunken in belly while still actively eating. Most of the time this will be tapeworm and 3 days of prazi will take care of it. But, sometimes, it can also be protozoan. To be sure, try 3 days of the General Cure in feed and look for the fish to dispel worms. if you pay attention, especially about 12- 24 hours later after feeding, you will be able to confirm a flatworm diagnosis.