What skin lesion is caused by Ich in fish?

Ich, or Icthyophthirius multifiliis, is a common parasitic disease affecting fish that presents as distinct white plaques on the skin and scales of the infected fish. These lesions are characterized by tiny, white cysts, which are the trophonts of the parasite that embed themselves in the skin and gills of the fish. The presence of these white spots is typically a result of the fish's immune response to the parasitic infestation, leading to local inflammation and the formation of cysts.

In contrast, red spots on fins are often indicative of different conditions, such as injuries or other infections, rather than Ich. Ulceration of the body surface and mucous membrane erosion may occur in various other diseases but are not typical presentations of Ich infestation. Instead, they usually indicate more severe systemic issues or secondary infections that can follow the primary condition caused by Ich. Thus, the characteristic white plaques on the skin and scales are the hallmark symptoms that lead to the correct identification of Ich in fish.