My fish have developed black speckled spots, like they got paint overspray on thier tails

Red Belly Pacu, only 2 of 3 fish in tank have this. What is it?

Its Black Spots, or diplopstomiasis.

Symptoms

-Small black speckles on body

-Flickering

-Small black smudges on fish

-Scratching against surfaces

-Very irritated

General Description

A disease caused by the larvae of parasitic genetic flukes, Zaccaria and Mascara. A common disease in earthen bottom ponds and lakes, this disease is caused by a parasite (larval trematode) that burrows into the skin of a fish causing the formation of a cyst approximately one millimetre in diameter. This parasite has a complex life cycle that requires fish eating birds or mammals, snails, and fish at different stages in order to survive.

In general, even heavy infestations of these parasites do relatively little damage to the fish. There is some evidence that heavily infested juvenile fish may experience excessive blood loss, physiological stress, and even death. Also, fish with heavy infestations on the eyes may be blinded.

The term "Black Spot" refers to the formation of small cysts in the muscle and skin around which the parasite lives. The cyst accumulates black pigment cells. These cause little harm to the fish. Young fish have growth problems if heavily infected. The first intermediate host is a mollusk. The fish is the second intermediate host and the final host is a kingfisher or heron.

Treatment

Black spot is generally easy to cure. There are a number of commercially available treatments and preventatives.