Ich Ichthyophthirius multifiliis
(Source)
Ich is an tiny ectoparasite that can harm fish if they succumb to it. If not cared for properly, it can kill fish.
It's important to note that Ich is always present, and there is nothing you can do to get rid of it. Fish are naturally immune to Ich, and only succumb to it when their immune systems or protective slime-coating have been compromised, or when they're stressed.
Think of it like a person with the flu. The flu virus will always be present, and our bodies have natural defenses in place to protect us, but external factors such as lack of sleep, a person who is already infected sneezing nearby, not washing hands, etc. can all cause the virus to get past our defenses and make us sick.
This is why you will commonly see fish contract Ich after traveling, if they're being bullied by other fish, and when they're living in poor water conditions. Also, if you have a fish like a Plecostomus, Loach, or certain catfishes, they will actually attach to other fish, and "bite" off their slime coating, exposing them to Ich.
Finally, just like if you moved a person sick with the flu into a house full of healthy people, introducing a fish with Ich into an established aquarium is a bad idea. And just like giving a healthy person flue medicine, giving healthy fish Ich medicine isn't a good idea.
It's a good idea to have a small tank on hand, one that's big enough for one or two fish, so that you can quarantine and treat sick fish away from healthy ones.
For freshwater fish, Ich can be treated in several different ways:
Chemical Treatment: Something like Ich Attack or ParaGuard are two that I personally recommend. Follow the directions on the bottle, it'll tell you how much to put in per gallon, and how often. Be careful, these two shouldn't effect anything else in your tank, but with chemical treatments you always run the risk of throwing off the chemical balance (usually Ph levels). Copper-based treatments like CopperSafe, while effective, are also really bad for invertebrates, like shrimp and crayfish. I would suggest avoiding copper if you have invertebrates in your tank.
Aquarium Salt: Adding aquarium salt to the water helps calm fish, providing them with certain nutrients, and removes nitrites helping fish to absorb more oxygen. I wouldn't recommend using aquarium salt with Plecostomus in the tank, as they can be sensitive to it, though some people have no problems. Also, aquarium salt is special, don't use other kinds of salt.
Temperature: Raising the temperature to about 86 degrees Fahrenheit will create an environment that the fish can survive in, but not the parasite. Don't leave the temperature at that though, as it will stress the fish out to be in there for long. It's all about who can survive it longer.
Aquarium Salt and temperature: I prefer this option as the salt helps the fish through the temperature rise more. Again, think twice about salt and Plecos.
For saltwater fish, the treatment is a bit more tricky. The first thing you want to consider is if your treatment will effect the live rock and any corals in your tank. In my opinion, for saltwater tanks, a small quarantine tank (with the same water parameters as the established tank) is a requirement. If you notice a sick fish, move it to the quarantine tank to protect the rest of your tank. There you can treat the fish without worrying about the medicine harming other fish or corals.
In a quarantine tank, us chemical treatments on the fish. Since you're treating the fish in a quarantine tank, you can use copper-based treatments like CopperSafe. I have not had the guts to try it, but according to the bottle, Ich Attack is reef safe. Meaning you should be able to use it in a tank with corals in it without killing the corals. I'm not going to say you shouldn't do anything like that, but I don't want to take responsibility for anything like that.
A UV sterilizer will help remove some of the parasites from the water. I've never used one, so I can't say much about them. They're expensive, and if you have the money for one, I think they'd be nice to have.
On top of the treatment, I would perform gentle water changes. Something like 10% every 5 days. Enough to take out a good chunk of the old water, but not enough to stress the fish anymore than necessary. Have a bucket of water prepared the day before with something like [Seachem Prime][18] for freshwater fish. For saltwater fish, use pre-mixed saltwater already heated to the right temperature to avoid any drastic changes.