It is a parasite, not a disease. It is often caused because the fish gets stressed, and mostly hits new fish because they are stressed during transportation (in a box), so really any new fish can have this. It often will show up a week to two weeks after the fish arrives, so if you got it at a fish store who does not quarantine for ich you have a chance of getting it. It can be deadly to fish but it is easy to treat.
The tank may or may not be "infected" with it. (They use disease words to describe it). The best thing to do is to treat the tank. Goldfish are cold-water fish, so raising the temperature may be a problem. You need to look that up (I have never had a goldfish). The typical treatment would be to raise the temperature to about 86 degrees. If you can't raise the temperature then the treatment period is longer. I have typically found this difficult. Most of the heaters I have cannot keep this steady temperature. The good thing is that once a fish gets, and is cured of Ich, they have a sort of immunity to it.
So raising the temperature will make the Ich bug's life cycle speed up. When it falls off the fish, the fish is actually not "cured." You have to vacuum your tank bottom and also add some kind of medicine made of formaline and malichite green. I use a fish medicine called Kordon "Rid Ich. "
You must dose based on the cubic square of the water, so measure the tank, then get the water level you will be using. Some people lower the water level so they can use a smaller dose because with a big tank the medication can get expensive, but remember this cure is going to take a while. Use the dosage on the back of the bottle (in the instructions) of the medication. Mine says 5ml. (1 tsp) per 10 gallons of water.
Every 24 hours you will take out 25% of the water, and put 25% of new water in. (with dechlorinator of course).
Every 24 hours vacuum the bottom of the tank (Siphon hose).
Every 24 hours add your medication for that new 25% of the new water. (the other medication is still in the bottom. Do not overdose.
Before medicating remove any invertibrates or snails.
Chose a medication that does not kill plants if you have them.
Watch the fish for distress-- gasping at the top-- difficulty breathing, clamped fins. (Have another tank ready to remove them quickly if that happens.
During this whole procedure, you must add extra oxygen to the water using an airstone, or a HOB with the water lowered. This adds a little noise to the situation. Make lots of bubbles however it is easier for you.
The majority of fish require only about 3-treatments. Which means 3-4 days of this treatment. With cold water, it may take longer.
The goal is to get the parasite as it falls off the fish and goes to the bottom of the tank where it has millions of babies that infect the other fish. When they are in that newborn baby stage they get killed by the medicine.
To get the medication out of your tank. Do plenty of water changes and also use a charcoal filter to take out any lingering meds. If you have raised the temperature, then begin to lower it slowly over several days.
What I generally would do is not to consider the fish (or tank) healed until they have been ich free for 2 weeks to a month. However so far this treatment works.
Cleaning everything after. Clean all equipment with hot water if possible. Do not use hot water on the tank. Water over 89 degrees F will kill ich so the water is not actually that hot. Another thing that kills ich is drying out. every tool you used needs to be clean and dry. With a syphon, you may want to have a separate one per tank.
There are other ways to treat ich. There is a way with salt, and a way with even higher temps. You need to read and find the best way for you.
One other thing is that you have time. The life cycle of ich is in 24 to 48-hour spurts.
You should always have fish medicines around.
I wish you luck!
