Ich

Treatment

There are a lot of different ways to treat ich. Because of how widespread and common it is, people have found some pretty neat ways of dealing with ich, some without any medication at all. Let's go through the scenarios. 

Single infected fish

Depending on whether the fish is freshwater or saltwater, a saltwater or freshwater dip will help get the fish ahead of the ich. Fish are much better at regulating their internal salinity than single celled organisms, like protozoa. Basically, osmotic pressure will cause water to flow to the place with less water and more salt to balance it out. Basically, water will rush out of the freshwater ich and into saltwater ich, killing each. Only leave the fish in the salt or freshwater dip for no more than 15 minutes or until the fish loses balance or looks otherwise distressed. Anything over 5 minutes is ideal, but do watch the fish for the duration of treatment. Repeat daily until 2 days after the ich clears up. 

If ich is not clearing up, move the infected fish into an isolation tank and treat with copper based medication. 

Multiple infected fish

If it becomes a widespread infection in a tank, it's best to just treat the whole tank. If you have invertebrates, this complicates it a bit. If you have no invertebrates that you care about, simply dosing the tank with your choice of ich medication will be the best option. If medicine is not an option, changing the salinity of the water (up or down depending on fresh or salt water) for a few weeks will work. Don't go above 1.005 in fresh or below 1.010 in salt. However, this is generally not the case, as many people like to keep shrimp and snails and even corals, and they will not tolerate either treatment well. There are two options at this point. The first is managing ich, the other is eradicating ich. 

Managing ich

The less labor-intensive way to deal with an ich outbreak is to manage stress in the fish. Some herbal remedies might be useful in this case. I've noticed that one such herbal remedy, herbtana, seems to help settle fish. Other things that are helpful are UV sterilizers, good food and nutrition, and proper maintenance of the tank. If all this is being done and the fish are in a mature tank, let the active ich infection pass and continue to do regular maintenance. The ich likely won't come back unless something else stresses the fish. 

Eradicating Ich

The safer and most effective, but more labor-intensive option is to eradicate the ich. This can be done in a manner safe for invertebrates and fish. First, you'll need to take every last fish out of the main tank and put them into a treatment/QT tank. Treat them with medication like normal. Next, you'll crank up the heat in your main tank to 80.6°F or more. This will speed up the life cycle of ich, so it dies out quicker. You will need to leave the fish in the QT tank for 6 weeks. Following that period, the fish can go back into the main tank and the heat returned to normal. Most invertebrates will tolerate the heat just fine, meaning that the ich will die out and the invertebrates will not.

Ich vs. Epistylis

In the aquarium community, there seems to be some misinformation regarding ich and epistylis in freshwater. While both are protozoans and often appear the same on the fish, there is no easy way to distinguish between the two, unless you are able to confirm via wet mount slides on a microscope. Some propose that ich nodules don't develop on the eyes of the fish, but that is simply not true. Ich will affect all the outer tissues of a fish, including the eyes. Another claim that isn't true is that ich is less deadly than epistylis. Again, these claims aren't true. It would seem that epistylis is far less deadly than ich, which will cause mortality if left untreated. Luckily, ich and epistylis are both protozoans, and are both susceptible to the same treatments. At the end of the day, the treatment is the same, and the symptoms are very similar, so it doesn't really make sense to even differentiate between the two. Assume that it is ich and treat as such.