Colour Change

by John
(Basingstoke UK)

I have two large fish in my pond. One is about 15 inches long and the other is about 12 inches long.

The smaller one had a lot of gold colour but has recently turned black, it also has an unsightly large body at the rear end.

The larger one has been black it's whole life, but is turning rapidly to gold.

Anyone know why?

John M.

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Grant's Reply

Hi John
Goldfish with a body length that size have reached, or are close to, the maximum size Common Goldfish can achieve.

If you are including the tail in the measurement, this can be confusing as Goldfish tail lengths vary greatly. As an example, a 6 inch Comet Goldfish can have a 6 inch tail. This is not a 12 inch Comet, it is a 6 inch comet. A 12 Comet will weigh 8 times the weight of a 6 inch Comet, which is a very large difference.

Starting with the easier question first, Goldfish usually turn gold within a few months of hatching, changing from a dull grey to black, to finally gold.

This process, depending on conditions, varies with each fish. Some never go gold, others can take years to change colour.

The fish that has changed back to black is unusual. Usually this occurs if there has been damage to the skin such as ammonia burn from poor water conditions, or a protozoan parasite attack such as Costia. Costia are prevalent in early spring.

Once the damage is repaired, the black fades away and the gold coloration returns.

I would be suspicious that something more may be going on with this fish that is related to the growth on the body.

The most common external tumour found in Goldfish are Neurofibromas, nerve sheath tumours that cause localized skin and fin lumps that can grow very large, fall off, and then regrow.

They are benign tumours and do not cause the fish any distress, but they can become a problem if they get so large as to hinder swimming.

If it is not a Neurofibroma tumour, I would need to see an image of the fish.

I would put the fish into a salt bath of aquarium salt at 0.3% solution (1 tablespoon per gallon, 3-4 grams per litre). Add 1/3rd immediately, 1/3rd in 8 hours, and the last 3rd 8 hours later.

I would leave the fish in the solution for a week to see if there is a change. The salt will deal with any protozoan parasites.

Be careful of water quality as more fish are killed in hospital containers than cured because water quality drops suddenly. Change the water frequently; if you don’t have a water test kit, daily.

Also, be careful of the fish jumping out of the container, as single tail fin varieties will jump when in unfamiliar surroundings.