What Happens if Your Goldfish Gets Big Black Dots on Him?

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Although traditionally a low-key pet, a goldfish can surprise his owner by developing big black dots, sometimes as a result of injury or infection, or simply as a developmental change. What happens to the spots depends on their cause. Black patches due to injury disappear, returning only if tank conditions or goldfish handling doesn't improve. But an infected fish keeps his spots until he's received treatment. Normal goldfish color changes might be temporary or permanent.

Clean Up Time

Black patches on a goldfish sometimes signify that his tank water was overdue for a change. Fish waste, uneaten food and plant debris create ammonia, a toxic chemical. If tank water isn't changed frequently enough, ammonia builds up and burns fish skin. When ammonia levels fall, healing skin turns black before returning to his normal color when fully healed. If you suspect your goldfish has suffered ammonia burns, remove 20 percent of his tank water every 10 days and replace it with water that has been left to stand for three days, to prevent black spots returning.

Blast From the Past

Poor-quality breeders or shippers might be the reason behind black spots on a recently bought goldfish. Knocking a fish against a hard surface and handling him roughly injures him, and bruises appear on his body and fins weeks or months later. Black patches due to injury are a good sign, because they show that the fish is in better health and his skin has started to heal. Bruises take two weeks or longer to disappear, the skin returning to its normal color.

Under the Skin

A goldfish in a pond, and, rarely, one in a fish tank, can suffer from a parasitic disease that causes his skin to develop black spots. Dark-pigmented parasites burrow under fish skin, causing irritating raised black spots. Other symptoms of infection include rubbing and flicking the body. An aquarium fish can be severely affected, but removing snails from the tank solves the problem. An outdoor fish requires professional diagnosis and treatment. Discouraging herons and other fish-eating birds from visiting ponds reduces the chances of re-infection.

A Fish Can Change His Spots

A goldfish can develop black spots as part of his normal color development. Goldfish colors include gold, white, red, silver and black, single tones or patterned in two or more colors. In the first year of life, goldfish change colors frequently; they can continue to change throughout their lives. An inexpensive goldfish is most likely to change color because his parentage isn't controlled, but even with a purebred, expensive fish, color stability isn't certain. Black patches caused by developmental color change might remain or change again later.

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