Most fish have parasites and they seldom affect the health of the fish except under unusual conditions. Most fish parasites have complex life cycles often involving birds, snails, and microscopic animals. Attempts to control them are usually futile and unnecessary except in controlled hatchery or fish farming situations. Larval tapeworms are common in the organs and body cavity of many fishes. Because internal organs are discarded when fish are cleaned, this parasite is often overlooked.
Black spot is caused by a parasite called a fluke which burrows into the skin of fish.
The fluke itself is actually a white color. Yellow grub is caused by flukes which burrow into a fish and curl up into a sac under the skin, in the muscle or in internal organs. These flukes are often found near the dorsal fins, but occasionally in the organs. When freed from the sac, flukes may be up to one half inch long. White grubs are a fluke that penetrates the skin and forms a sac in the kidneys or liver.