Use a camera with its flash on, to photograph the fish after lights out. That will show up velvet and you can post the pictures on here for us to check them.
Make sure the pictures are in focus
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Salt baths are useless for a couple of reasons.
1) You stress and damage the fish each time you catch it and move it, and the fish draws in salt and then after the bath is draws in water (osmotic pressure).
2) Any diseases are in the main tank and when you put the fish back in the main tank, it gets infected again.
You just add salt to the main tank and leave it there for a couple of weeks to kill the diseases in the tank.
Make sure the pictures are in focus
-------------------
Salt baths are useless for a couple of reasons.
1) You stress and damage the fish each time you catch it and move it, and the fish draws in salt and then after the bath is draws in water (osmotic pressure).
2) Any diseases are in the main tank and when you put the fish back in the main tank, it gets infected again.
You just add salt to the main tank and leave it there for a couple of weeks to kill the diseases in the tank.