[daniella3d]

I doubt it. Your problem started weeks ago. If it was marine velvet all your fish would be dead my now. It's way too long to be marine velvet.

Unless you ID it under a microscope you will not really know what it is. I would do this on the first dead fish, that would probably save the rest if the right treatment is applied.

For exemple, if you have brooklynella in your tank, copper won't have any effet on it and you must use formaline. Brooklynella is a good fish killer as well. This is why it is so important to properly ID the parasite to begin with. Loosing time with the wrong treatment is a sure way to kill your fish.