Ichthyobodo and White Patches on Fish's Skin
Symptoms of Ichthyobodo necator (formerly called Costia necatrix) in fish is characterized by flat gray or white patches on the fish's body. It looks different from fungus, which usually forms wool-like tufts. Infection with Ichthyobodo is smooth in appearance. The smooth, flat, white patches are caused by excess mucus production by the fish in response to a skin irritant.
The Ichthyobodo necator parasite is a protozoan that has two life stages. One is a free-swimming stage. In this stage the parasite swims until it locates a host. When a suitable host is found, such as a fish, it attaches to it.
Once the parasite attaches to the fish it is now in the parasitic stage. During this stage it feeds off the fish's skin. It also multiplies on the fish's skin by cell division.
Some species of fish are more susceptable to Ichthyobodo infection than others.
There are two other fish diseases with the same symptoms as Ichthyobodo necator. These are Chilodonella and Trichodina. They are also external parasites and treatment for each of these is the same as for Ichthyobodo.
Treatment for White Patches on Fish
Before you attempt to treat your fish with medication you should check your pH because an extremely high or low pH may cause a fish to produce excess mucus.
If the cause of your fish's skin cloudiness is due to Ichthyobodo or one of the other 2 parasites mentioned, turning up the temperature to 83-86 degrees Fahrenheit or 28-30 degrees Celsius will help to kill it. Unfortunately, in unsuitable conditions this parasite can form a cyst that is resistant to destruction by heat and other methods. This means that it may reinfect your tank at a later date, such as when the temperature is turned back down to normal.
Other treatments that may work are copper sulfate, acriflavine, formulin-based medicines, and medications that contain malachite green. The medication Maricide by Mardel states that it treats a variety of diseases (ich, velvet, and other external parasites) and so it may work to treat your fish if its problem is caused by external parasites. It contains malachite green as the active substance.
Keep in mind that using medications in your tank often has unwanted effects as well as the intended effects. For example, malachite green may kill off the nitrifying bacteria in your filter as well as your plants. Medications with formulin in them cause cancer in humans (if you eat it or have prolonged contact with it). Just putting some into your fish tank is probably all right.
Because there are many aquarium pathogens that can affect your fish, it is often difficult to know what the cause of the trouble is. Unfortunately, without examining the parasites that are infecting your fish under a microscope you won't know for certain whether or not your fish has one of the 3 parasites mentioned above or a bacterial infection or even a fungal infection. Fungal infections usually are not flat and smooth and so you should be able to rule this out pretty quickly. If you have livebearing fish, especially female guppies that have patches of white on them this may NOT be due to Ichthyobodo. For example, female guppies are especially susceptible to a bacterial infection that looks very similar. This infection is caused by columnaris and it can be treated with a medication like Maracyn or Maracyn-Two. You could also use Melafix by API. It also helps cure bacterial infections in fish, however, it isn't as strong as Maracyn or Maracyn-Two.
Columnaris is becoming increasingly common in freshwater fish, whereas Ichythyobodo isn't all that common. If you look at the disease descriptions given in a lot of older publications you may think your fish has Ichthyobodo when it really has columnaris. Also see the article Columnaris.
A lot of freshwater diseases can be helped or prevented by adding aquarium salt to the tank. I think it is best to try adding salt and turning up the temperature for a lot of conditions before resorting to the stronger fish medications. You can buy aquarium salt specifically made for freshwater fish, or you can buy uniodized salt at the grocery store. Don't use salt with iodine in it (iodized salt)!! It will kill your fish. I usually use about 1 tablespoon of aquarium salt for every 5 gallons of water. Keep in mind that if you have a 10 gallon tank that your rocks, heater, and other items in your tank take up some space and so you may not actually have 10 gallons of water. Also, salt doesn't evaporate and so when you replace water in your tank don't add more salt to it. You can, however, add more salt when making water changes (as long as it doesn't exceed the recommended salt level). Keep in mind that some fish don't tolerate extra salt as well as others (such as Bettas).
Remember, if possible it is always best to try to narrow down the cause of your fish's illness before using any medication. The fewer medications used in your tank the better.