I have used about 3/4 tbs per US gal during the treatment of newly imported Hyancistrus sp L66 and L318,also Panaquae L204 badly infected with Chilodonella. Other drugs being used at the same time were malachite green, flubendazole, praziquantal and metronidazole.
Bad Chilodonella infestations can inflame these fishs' skin badly and salt seems to promote healing as the parasite cysts leave.
The duration of the salt treatment was usually 7-10 days. There happened to be a few Ancistrus cf. cirrhosus juvenile in the treatment tanks incidental to the use of the tanks as treatment tanks. I suspect even a black water Ancistrus sp could tolerate brief exposure to salty water. I have used this same regime on newly imported young wild Heckel Discus, another true black water fish, without any losses.
At least with these species were able to tolerate salt as long as I used it. I also dipped them first in about a 0.3% salt solution for 20 minutes before placing them in the treatment tanks.
There is no doubt that plecos can tolerate small quantities of salt in the water. 0.1% is certainly feasible (1g per liter, about 1 tbsp per gallon).
However, this should only be done short term, as part of a treatment for parasites etc. These fish do not normally live in salty or hard water in nature (there are exceptions, but as a simple "one rule applies to all", the answer is "not").
Now, I know that you need to be careful with meds. like Quick Cure which contain formalin and malachite green, but what about meds. like Jungle Parasite Clear with praziquantel, metronidazole, and acriflavine?
Luckily, my plecos are healthy....just wanting to expand my knowledge on this topic.