Treatment of Freshwater Ich, Ichthyophthirius.
It is believed that ich is present in all aquariums. Fish that have the disease exhibit small white dots about this size of a grain of salt. It is very contagious and it is fatal.
The best protection is a healthy tank with water changes done weekly or every other week with a gravel vac in the amount of 20-25% of the tank volume. Ich thrives in temperatures between 55-70°, making gold fish extremely vulnerable. Heating a tank to 72-73° is not too warm for gold fish and the susceptibility to ich will be dramatically reduced. A sound practice in fish keeping is to keep new fish in quarantine 4-7 days before adding them to your tank. The stress from being shipped from the wholesaler to the retailer, the stress of living in the retailers tanks which may be overcrowded and poorly maintained and the stress of being bagged up and taken to your home can weaken a fish and make it most vulnerable to ich and other ailments. Rather than introduce a sick fish to your aquarium, it is better to quarantine it until you know it is healthy.
When sound fish keeping practices are not enough, fortunately ich is also very easy to cure. Freshwater ich should not be confused with marine ich, Cryptocaryon irritans. Salt will weaken freshwater ich, obviously marine ich, being in salt water already, this is obviously not the same treatment.
First step:
20-25% water change with gravel vac. Most likely your poor water conditions contributed to the outbreak of ich.
Second step:
Raise temperature (no more than 1° per hour) to 85°.
Third step:
Add aquarium salt (not table salt) in the amount of one rounded tablespoon per 5 gallons. If you have scaleless fish such as loaches, catfish and "algae eaters", reduce that to one rounded teaspoon per 5 gallons, as they don't tolerate salt well.
http://www.aquariumpharm.com/en_us/productCategory.asp?categoryname=WaterConditioners
Fourth step:
After 24 hours, a second 20-25% water change. Add more salt. If you took 5 gallons out in the change, put another rounded tablespoon in.
Adjust for your situation.
Fifth step:
Wait 24 hours.
Sixth Step:
A third water change of 20-25% and replace salt removed.
If this does not cure the fish, and no signs of improvement show up yet, you may want to use a commercial ich curative, all of which require removal of the carbon from your filter, or the carbon will filter out the medication. You can continue with the cleaning process and maintain salt levels. Brackish water fish such as mollies need to have some salt in order to maintain good health. Most other fish benefit from a little aquarium salt as well.
One of the most common is copper sulfate medications like Aquarasol http://www.aq-products.com/APpro/aquarisol.htm
In more extreme cases, Malachite Green is very popular.
http://www.aq-products.com/APpro/quickcure.htm
In the case of scaleless fish such as loaches and catfish, a formaldehyde based Formalin
http://www.aq-products.com/APpro/formalin.htm
Be extremely careful with Formalin doses, many are super concentrated containing 37% formaldehyde, as 1 teaspoon treats 90 gallons.
Some are pre-diluted like Formalin-3, where the dosage is 1-2 teaspoons per 10 gallons. Formalin will kill the bacteria in your filter that break down the waste,
http://www.novalek.com/kordon/formalin/index.htm
Some products contain a combination of these medications. Read the labels and know what you are putting in your tank.
Good Luck