Parasitic Fish Diseases

Parasites
There are two ways to know that your fish have parasites. Very tiny white spots (see photo below) and when the fish scratches on the rock.
The first is somewhat easy to cure. If the tank does not have any other medications, especially copper, then try Marine Max by Tropical Science. It's a biological additive that's 100% reef safe and does not alter water quality. In the case of sever infestation, the entire 2 part bottle can be added / 100 gallons. (Shut off UV sterilizers, remove carbon and turn off protein skimmer.) If you use Marine Max on a weekly basis (add a small equal squirt when adding a new fish to your tank each bottle), it works even better as a preventative.

When a fish scratches itself against rocks or gravel, it is usually a parasite attached on or near its gills. Marine Max often works but in some cases the little critters may be more persistent.
We have found Prazipro will take care the problem. It is reef and coral safe. 1 ounce will treat 200 gallons. (Shut off UV sterilizers, remove carbon and shut off air to the protein skimmer.)

When adding a new fish to your tank and you don't have a quarantine tank, here a fresh water bath works well.
Using a plastic bag or baggie, add a quart or more of water but be sure it is sufficient for a sick fish to swim upright. Add and mix 1/4 teaspoon of baking soda (Arm and Hammer) to raise the pH to 8.3. Then let the bag float in your tank until the temperature matches your tank. Net the fish and put him in the fresh water for 3 to 3.5 minutes. Time this - do not guess. The fish will not like this and smaller fish will like it much less than larger ones. If your fish manages to swim a little when you disturb him (from the outside of the bag) he is handling the bath OK. When time is up, tip the bag over and release the fish and the water. Release him early if you get really worried.

Copper as a cure
If you have recently used some form of copper as an ick (parasite) cure, biological remedies will not work as they are also killed. Other drawbacks of copper include:
1. Your good bacteria are reduced in numbers. This results in a new tank cycle with the return of ammonia and nitrites. Mother Nature will fix the problem if you have patience. If ammonia and nitrite get dangerously high, change 50% or more of your water. Prime by Sea Chem will do the same thing instantly. Have some on hand when using copper treatments or use NitroMax by Tropical Science or any bacteria booster type.
2. Angel fish are not tolerant of copper medications. Cupramine by Sea Chem has worked for us in low doses with no side effects. The most common thing seen in angels is a secondary infection of a virus called lymphocystis. It appears as bright white growths on the very edges of the fins. Because viruses cannot be treated, you can only allow Mother Nature to take her course. Just let it be and it will go away. In extreme cases the fish is removed from the tank and the growths are removed surgically. Advanced cases can be fatal.
3. Copper is stressful of the kidneys, liver, and other internal organs. Over dosing copper can kill. Testing isn't necessary if you follow directions.
4. Secondary infections can occur because of copper. Most common are bacterial infection. Open sores, often bloody in appearance, are what you should look for. Treat with erythromycin by adding 200 mg / 50 gallons to the tank and mixing powdered antibiotic with their food. Some algae eaters can be treated by soaking Nori or seaweed in a very concentrated solution, letting it dry and then feeding it to the fish. Internal treatments should be used in reef tanks. Small doses of erythromycin are harmless to corals. This is how we eliminate red "slime algae" in reef tanks.
5. Copper renders tanks uninhabitable to corals and inverts. It can be removed using EDTA which is a heavy metal remover. Only in this way can you guarantee complete removal - other absorbents are not 100%. Copper does absorb into your gravel and rock, this is why you need to re-dose during treatment. Use a snail as the ultimate test for copper.

What is Black Spot Disease?
Typically a "disease" that is most commonly associated with Tangs and Surgeonfishes, but one that can be contracted by other species as well, what is referred to as Black Spot Disease, Tang Disease or Black Ich is actually caused by an infestation of tiny Genus Paravortex member turbellarian flatworms.
Although parasitic organisms that are much less dangerous and life-threatening, as well as rather easy to get rid of compared to other ich diseases such as Oodinium, Cryptocaryon, and Brooklynella, nonetheless it is a problem that needs to be treated upon recognition to eradicate infected fishes of these parasites.