This species, like all scaleless & knifefishes, is very sensitive to chemicals. They prefer soft, slightly acid water around pH 6.8, but are not too picky. medicines or metals in the tank can be very toxic to them. They also are not big fans of salt for the same reason, so while it may have it's use for therapeutic reasons, keeping them in salted water longterm is not a good idea. The tablespoon per 5 gallons is about the limit you'd want to risk.
Personally, I think the upside down cat is the problem. They like to eat the slime off of slimy fishes, and black ghosts are decidedly slimy, with skin thin enough to be irritated by such annoyances.
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