This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Symptoms: Fish look like they have little white salt grains on them
and may scratch against objects in the tank.
White spot disease (Ichthyopthirius multifiliis) is caused by a
protozoan with a life cycle that includes a free-living stage. Ich
grows on a fish --> it falls off and attaches to gravel or tank glass
--> it reproduces to MANY parasites --> these swarmers then attach to
other fish. If the swarmers do not find a fish host, they die in about
3 days (depending on the water temperature).
Therefore, to treat it, medicine must be added to the display tank to
kill free-living parasites. If fish are removed to quarantine,
parasites living in the tank will escape the treatment -- unless ALL
fish are removed for about a week in freshwater or three weeks in
saltwater systems. In a reef tank, where invertebrates are sensitive
to ich medications, removing the fish is the only option. Some people
think that ich is probably dormant in most tanks. It is most often
triggered by temperature fluctuations.
Remedy: For most fish, use a medication with formalin and malachite
green. These are the active ingredients in many ich medications at
fish shops. Some products are Kordon's Rid Ich and Aquarium Products'
Quick Cure. Just read the label and you may find others. Check for
temperature fluctuations in the tank and fix them to avoid
recurrences. Note that tetras can be a little sensitive to malachite
green, so use it at half the dose.
Use these products as directed (usually a daily dose) until all of the
fish are spot-free. Then dose every three days for a total of four
more doses. This will kill any free-swimming parasites as they hatch
out of cysts.
Another remedy is to raise the tank temperature to about 90 deg F and
add 1 tsp/gallon salt to the water. Not all fish tolerate this.
Finally, one can treat ich with a "transfer method." Fish are moved
daily into a different tank with clean, conditioned, warmed water.
Parasites that came off of the fish are left behind in the tank. After
moving the fish daily for a week, the fish (presumably cured) can be
put back into the main tank. The disadvantage of this method is that
it stresses both fish and fishkeeper.