If you notice that your fish have white spots on them, they're probably suffering from ick. Ick is the most common parasite associated with aquarium keeping. It occurs in fresh water and salt water. It's usually brought out by a change in water temperature. That's usually what brings the parasite out of its dormant life cycle into the free swimming stage, where it can latch onto the fish. These parasites are actually eating the slime off the fish. They're eating the skin of the fish. They're easily controlled with a lot of different types of medication.
So when you first see some white spots on your fish, don't freak out. Don't remove the fish from the tank. The ick is already in your aquarium. The fish's immune system is capable of fighting off the ick. Like a lot of times you'll see it's the newest fish or the fish that are stressed out or not eating that much, those are the fish that succumb to ick, and the fish that are very healthy don't have any white spots on it. It's just like humans, where ten people could be exposed to a flu bug, but only half of them come down with it, because they're immune system is compromised. So the fish's immune system goes a long way to keeping the ick at bay.
You want to promote a healthy immune system by keeping the water as clean and as close to the natural conditions that the fish require as possible. You also want to feed them a balanced diet. You can put some garlic in the fish's food, which helps to control parasite outbreaks in aquariums. And then if you do have ick and, and you see the white spots, go to your aquarium store. Tell them the type of fish you have, because certain medications are lethal to certain types of fish, like scaleless fish don't do well with a lot of the ick medications.
Once you buy an ick medicine, you want to remove the carbon from your filter, as the carbon will remove the medication. You should also bump up the temperature in your tank. The parasite is only treatable when it's in the free swimming stage, and when you first put medicine in the aquarium, the medicine is toxic and really potent for the first day or two, and then it loses its potency. So you want to expose as many free swimming stages of the parasite to that medicine when it's strongest. By bumping up the temperate, you're speeding up the life cycle. You're making the medicine treat the parasite a lot more effectively. But you want to run that medicine for 10 to 14 days to really eradicate the parasite, and then you could put fresh carbon in and do a water change to remove the residue, or the leftover, medicine. Again, get the advice of your local store, but jump on it right away, because ick is lethal, and it can wipe out a whole tank, so you definitely want to take it seriously.
Another way to prevent ick is by buying fish from tanks that don't have any signs of ick or disease. If you're buying fish out of central systems and you see sick fish in that water, chances are there are a lot of free swimming parasites that you can introduce to your tank. You can also quarantine your fish before you put them in your tank. You can give freshwater fish a salt bath and saltwater fish a freshwater bath. The theory behind that is the osmotic pressure change will cause the parasites to fall off the fish. So if you have a saltwater fish and you expose it to fresh water for four or five minutes before putting it in your aquarium, the parasites will fall off the fish, and you can prevent introducing parasites into your aquarium that way.
A lot of information on this topic, a lot more than I can talk about right now. But if you see ick on your fish, you definitely want to tackle it right away.