To be honest iI think I would add a parasite med like clout, same as you say something not right if this is happening not just after a water change.
As to be honest don't think you have anything to lose, as the fish are going one by one.
Clout
Manufacturer: Aquarium Products
A very strong and effective medication for parasitic and protozoan infestations. Treats: Ich, Hydra, leeches, Planaria, Epistylis, Trichodina, Hexamita,Tetrahy-mena, body fungus, digenetic flukes, parasitic copepods, monogenetic flukes and anchor worms