Planted Tank Guru
Join Date: Dec 2017
Location: Elk Grove, California
That white "saddle" in front of the dorsal of the fish that died looks very characteristic of Columnaris. But, you say they are acting like/you visually saw a parasite on the gill. Are you sure it wasnt heavy mucus and not a parasite? Columnaris can also attack the gills-- it is known by its symptoms as "Saddle-back" disease, "Cotton-wool" disease, and "Bacterial Gill" disease. But, there are 4 strains so not all of these symptoms show up in all strains.
Are the fish rubbing on objects, backing up as if trying to escape some irritant, gasping for breath? It could be Gill Flukes. (Gill flukes are not visible to naked eye, however). Which, in that case you will need to treat with the Prazi-pro for 21 days.
I know you said that your parameters were "stable", but- what is your nitrate level?
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