The common aquarium disease ick looks like salt crystals. However, the white specks on infected fish are actually tiny cysts, caused by parasites, not salt crystals. You can actually treat ick with salt crystals to control the parasites.
What is Ick?
Ick is short for Ichthyophthirius multifiliis, the parasite that causes the disease. This parasite is a protozoan with a complex life cycle, including parasitic and free-swimming stages. Each generation gets exponentially larger, so you must take quick action to prevent ick from overrunning an aquarium. Additionally, all treatments for ick have to target the relatively vulnerable free-swimming stages, since the cysts are actually very hard to treat directly.
The biggest sign of ick are the presence of tiny white cysts, which look like someone sprinkled salt on the skin of your fish. Additionally, infected fish may show behavioral changes. Fish with ick may scrape themselves against aquarium decorations to try and dislodge the parasites. Also, be on the lookout for velvet, a similar disease caused by a related parasite. If you see the same symptoms, but cysts that look more like powdered sugar with a yellowish tint, it's velvet; fortunately, it has the same treatment.
Certain stages of the parasites' life cycle are vulnerable to even trace amounts of salt in the water. To treat, dissolve aquarium salt, rock salt or kosher salt in a bucket -- 3 teaspoons of salt per gallon of your aquarium's water volume. Over the course of three hours, add the salted water to the aquarium. Premixing the salt in a bucket and slowly adding it to the main tank prevents shocking your fish. You should also slowly raise the aquarium's temperature to 82 to 86 degrees Fahrenheit, since ick has a harder time surviving at higher temperatures. This typically clears up ick in about a week. After which, perform a 25 percent water change to restore the regular salinity.
Unfortunately, many aquarium fish cannot tolerate salt and high temperatures, so you will have to resort to aquarium antibiotics. In the U.S., you can procure these at most pet shops. However, in the UK, you may require a veterinarian's prescription. When treating, remove carbon and zeolite, since these absorb antibiotics. Typically, these treatments also take about a week.