Thank you to everyone who answered me with advice. The treatment has been done and all seems to be well in the tank. I opted for Pimafix. I was hesitant to use antibiotic (Maracyn 2) on fully healthy fish. At the start of the treatment I happened to be in an aquarium focused independent pet store, whose owner's advice I respect. He advised aquarium salt at 1 tablespoon / 5 gal
. I asked about Otos, Tetras, and plants and he said that he uses salt at that concentration in all his tanks. I added the salt and completed the treatment. All seemed well. At the completion of the course of pimafix, I did a 25% water change but did not top up the salt. I was concerned about the effect on the plants, not that any ill effects were evident. I sat on the fence until the next 25% water change, but since all seemed well with plants, fish and snails, I calculated the salt necessary to bring the concentration back up to the recommended level and will maintain it there.