Goldfish Getting Black Lines and Dots

by Cristina
(Florida)

Hello, I got an aquarium set up 2 weeks ago. It is 36 gallons and has 3 small goldfish with a filter for a 40 gallon tank, an oxygen pump, 3 real plants, and bogwood.

Each goldfish came from a different pet store. One of them had an anchorworm and infection which healed once the anchorworm was pulled and paraguard was added to the tank. But then the smallest fish started to get some black lines and dots along the tail and sides. I read that it could be caused by ammonia burns.

I took some pictures to a local store that specializes in fish and they think it is just the fish changing color.

Is the fish sick?

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Grant's Reply


Hi Cristina

There are several issues that need to be clarified.

Is the aquarium cycled, meaning has it gone through the nitrogen cycle?

Have you tested the water for ammonia, nitrites or pH? This would have given you the answer regarding ammonia.

Were the fish quarantined and treated for parasites (other than anchor worm)?

When a new aquarium is set up, it is usually at the two/three week mark that things start going wrong.

I think your local fish store was half right, I think the line on the fish is just marking that may or may not stay.

The other black mark is caused by skin damage that the fish is repairing.

With three small fish in a 40 gallon tank, it has a light bio-load. I don't think ammonia would be present, but you will need to get a cheap water test kit (not test strips) and test the water to confirm.

If ammonia is present, a large water change, (using water conditioner), will need to be made immediately, and daily 25% water changes made until no ammonia is detected when the water is tested.

Because you have used Paraguard, and the fish developed black areas, I believe the fish was diseased when you got it, and is now repairing the damage.

If all the fish are feeding well with all fins held erect from the body, particularly the dorsal fin, I don't think you have a problem. The black patches should gradually fade away, being replaced by the fish's normal color.