Then uneaten food is the underlying cause of the fungus. Your BN will help a little with that problem. Might think of adding a small school of agressive eaters like Danios, larger tetras or
Barbs. Aquarium Salt kills fungus, but is not particularly good for inverts or plants.
The best weapon is a siphon coupled with WC
's. Rake the substrate vigorously with the siphon. I've never used salt in my tanks, but many member regularly do. Advice will likely be coming.