according to this part
Marked the important part in red.
Marine Velvet Disease
As I understand it Ich is a lot more on the skin, where velvet primary effect the gills. If a lot of marks are visible on the skin, with velvet you should have the fish gasping for air. Slime layer also increases.
No marketable increase in mucous or no gaping for air with a lot of spots on the fish, more likely Ich.
Please feel free if anybody disagree.
After all. I'm not a marine biologist.
Symptoms
The symptoms of marine velvet usually involve the skin and lungs. Mild infections will usually only infect the gills and the fish may show minimal symptoms. As the infestation becomes more severe, the gills will become inflamed, bleed, and the lung tissue will begin to die. The fish will show signs of irritation and distress, with rapid breathing and lethargy. As the inflammation increases, the fish will lose its ability to transport oxygen across the gill membranes resulting in a fish that shows symptoms of suffocation, and if treatment is not initiated, death will often result.
The skin is the site of attachment for the organism and in severe infections, small gold-colored spots will cover the skin, which can progress to create a "velvet" appearance which gives the disease its name. By the time the gold-colored velvet appears, however, the gills may be so infected that treatment is usually too late. Many fish die from this disease without ever showing any visible skin changes. It may be possible to visualize early forms of the infection on the skin by using indirect illumination. This works best on dark fish and can be done by shining a flashlight on the dorsal surface of a fish in a darkened room. Viewing infected fish against a dark background may also be helpful.
Marked the important part in red.
Marine Velvet Disease
As I understand it Ich is a lot more on the skin, where velvet primary effect the gills. If a lot of marks are visible on the skin, with velvet you should have the fish gasping for air. Slime layer also increases.
No marketable increase in mucous or no gaping for air with a lot of spots on the fish, more likely Ich.
Please feel free if anybody disagree.
After all. I'm not a marine biologist.