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Angelfish may not do well with other fish. They tend to be territorial, and may attack and even eat smaller fish. If you introduce other fish, stick to other angelfish or fish of a similar size. If an angelfish becomes diseased, you will need to talk to a vet or a pet store worker about treating the illness. This is especially important if you have other fish in your tank. One sick fish could infect the tank.  Excessive slime and clamped fins can be a sign of a very serious disease called angelfish virus. If you suspect your fish has the virus, it may need to be put to sleep as there is no cure. White chalky feces, a lack of appetite, and weight loss can also indicate disease. There is also a common disease called ich that causes white spotting due to parasites. It can be treated easily with medication, so keep ich medication on hand when you have angelfish. If an angelfish is showing signs of disease, remove it immediately and place it in a quarantine tank. Seek the care of a veterinarian to discuss treatment options or ask at a local pet store. Do not put the fish back into the tank until the signs of disease have passed, as you do not want disease to spread.
Use caution when placing angelfish with other fish. Watch for signs of disease. Quarantine sick angelfish.