…both fresh and saltwater is Ich (Ichthyophthirius multifiliis) or white spot. Others frequently found include Anchor Worms, Black Spot, Costia, Gil Flukes, Hexamita, Fish Lice, and Velvet. 

The best prevention against disease is to keep your fish healthy in a stress-free environment. A varied diet, stable temperature, and good water parameters are key. If they do get sick, use this selection… 

Saltwater ich (ick), or "white spot disease" is one of the most common diseases that infect marine fish in home aquariums. It is caused by Cryptocaryon irritans, and is similar to freshwater ich. 

Many commercial Ich treatments utilize copper to kill the Ich parasite. This is fine in fish only aquariums, but copper is toxic to aquatic invertebrates like snails, crabs, shrimp and crayfish.