PlanetCatfish.com

Ok, let me start by saying this. I don't want this to start the discussion about using/not using salt in your tank (again). It seems that you can get just as much advice that says that you SHOULD use salt as says that you SHOULD NOT.

I have one question concerning salinity though... If you should take a fish from a tank with a certain ammount of salt in it and put it in a tank with a higher (or lower) ammount of salt in it. Could this cause symptoms of popeye on the fish? And if so is it the higher or lower salinity that causes it? (shouldn't one of the two push the eyes in instead? )

I have seen this a couple of times... fish developing popeye-symptoms shortly (minutes) after being moved to a new tank. (one case involving one of our customers bought a couple of Uaru amphiacanthoides and took them home, put them in his tank with conditioned RO water - Popeye within minutes, in this case he cured it by adding salt)

TalenT wrote:I have seen this a couple of times... fish developing popeye-symptoms shortly (minutes) after being moved to a new tank. (one case involving one of our customers bought a couple of Uaru amphiacanthoides and took them home, put them in his tank with conditioned RO water - Popeye within minutes, in this case he cured it by adding salt)

The Pop-eye was probably just the easiest way to see that there was Oedema taking place in all the tissues of this fish. Assumedly the customer had water with far lower dissolved solids than were in your stock aquaria, and the physiology of the two Uaru was adapted to dealing with the demands of living in your water. Once in your customer's aquarium, the osmotic pressure was far greater and so they were unable to remove the water as quickly as it was passing through the cell membranes.
Pop-eye can have many and varied causes, and can be linked to generalised oedema or can be a symptom of something restricted to the eyes. In cases where it is due to generalised oedema, salt might help to disguise the symptoms but it wouldn't address the underlying cause (in the case of the Uaru you mention the salt could be gradually diluted and the physiology would adapt to this slower change). If only the eyes of the fish were affected, then salt at therapeutic levels might help to treat the infection, but then again it might just add to the stress on the physiology of the fish.