fungus and bacteria present in all aquariums cause angelfish fin rot.

milk like coating on the fish, redness in gills, fungus-deteriorating split fins.
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Antibiotic solutions such as Myxazin, Melafix or Gentian Violet may help your fish recover quicker. Sometimes fin rot can be compounded by fungal infections, worsening the problem.
Cotton fin fungus is a condition that typically affects aquarium fish with weak immune systems
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MELAFIX smoothes the transition from store to home by reducing the risk of your fish developing a disease. The fish you bought may already carry a disease. Disease is often difficult to detect, particularly in its early stages. To treat these fish and reduce the risk of contaminating other fish use MELAFIX, an all natural treatment. Bacterial disease destroys fish tissue. MELAFIX, heals open wounds abrasions, treats fin and tail rot, eye cloud, mouth fungus and promotes re-growth of damaged fin rays & tissue. Will not adversely affect the biological filter, alter the pH, or discolor water. Safe for reef aquariums and live plants. For use in fresh or salt water aquariums. also dilutes the amount of bacteria/fungus in the water and can help the fish combat the fin rot.
Photo provided by FlickrAn infection of fungus often follows another infection or injury, usually on a fish's lips or fins
Photo provided by FlickrMy fish have a fungal infection and fin rot so I set up a emergency qt tank for them
Photo provided by Flickr
Since fish have no hands, they use their mouths to touch everything, to take food, and to fight (hopefully not). The most frequently injuries take place on their mouths. It is easier for the bacteria to get a foothold there when there are open wounds. When this occurs, the fish’s mouth can at first look like having had some white lipsticks. When it gets worse, fish lips can be covered by what looks like white or grayish mold or cotton. In more severe cases, you can literally see the fish lips are rotting away or falling apart. Sometimes columnaris can also infect the fish’s gills and fins. In those case, they are called gill rot and fin rot/fungus.Second Treatment: SulfaPlex vs. Fish Mouth Rot
I picked first, another antibiotic and anti-fungal medication. It contains 67% sulfathiazole which was said to be highly effective treating bacterial infection. Fish mouth rot and fin rot were both listed in its description as treatable diseases. Due to bad luck, it did not work for me. Just like ParaGuard, it did not affect the biological filtration.Cotton fin fungus is a condition that typically affects aquarium fish with weak immune systems. The condition also goes by the name “Cotton Wool Disease”, characterized by the cotton wool growths on different parts of the affected fish.When a koi has cotton wool fungus, the fish develops what looks like a fine-textured fur coat over its body that’s actually mold growing on the skin. Sometimes the fungus looks like a pale-orange or ivory-colored layer. As the disease progresses, the fungus grows longer, cotton-like tufts.Third Treatment: KanaPlex vs. Fish Mouth Rot
I finally ordered . It was a broad spectrum antibiotic, and it said to be able to treat various bacterial and fungal fish diseases. Once again, both mouth rot and fin rot were listed under its treatable fish diseases in the product description. Furthermore, it was said to be easily absorbed through both fish skin and gills. So ingestion is not required. After just a week of treatment, I could tell there was noticeable improvement by the appearance of the fish lips. All visible symptoms were gone within two weeks. To be on the safe side, I continued the treatment for another two weeks although it was probably unnecessary to do that long.Effectively treats fungus (Saprolegnia spp. & Achlya spp.), mouth fungus (Cotton Mouth Disease), Body Slime & Eye cloud and Fin & Tail Rot infections. Quickly controls contagious fungus and secondary bacterial infections. Helps prevent the growth of fungus on fish eggs prior to hatching. For use in freshwater aquariums. Note: This medication will cause a slight discoloration of water which can be removed with activated carbon.