Are there diseases that only happen in salt tanks?

There are plenty that only occur in saltwater tanks (or even brackish), and many that occur in both are a result of a different parasite, or bacteria, or other aspect.

Yes, there are quite a few diseases that do happen in just saltwater tanks. Some of them are more common than others and it is wise to know about the more common ones as well as the less common ones.

Some of the more common ones that I can think of off the top of my head are

Marine White Spot Disease
Clownfish Disease
Flukes
Bacterial Infections
Swim Bladder Disease

I know there are more than this in terms of common diseases, but I hope these help a little.