Goldfish Growth Differences

by Dayton K.
(Indianapolis Indiana)

I have three goldfish in a 250 gallon tank. It's huge two of my goldfish are almost ten inches but the third one doesn't seem to be growing.
I've had him for almost a year, how can I solve this problem?

Grant’s Reply

Hi Dayton

Goldfish growing at different rates usually means the amount of food being consumed is different for each fish.

This shows up very clearly with Goldfish fry if a spawn is underfed. A few individuals will get most of the food and their growth rate will rocket ahead of the rest.
If fry are fed all they can eat, there is very little size difference between them, even when the spawn numbers several thousand.

With your fish we know the food type and quality is going to be the same for all fish so we can eliminate that as a possible cause.
With some fish species, larger fish give off a hormone that stunts the growth of smaller or younger siblings. With Goldfish this isn’t thought to be a problem, and with the size of aquarium you have, your fish won’t be stressed enough to give off any stress related hormones.

So, having eliminated environment, the cause is going to be either not enough food, disease or something physically different with the smaller fish.

Physical Problems

I am assuming all your fish are the same type so they can compete equally for food.
Many goldfish keepers have an aquarium full of single tailed fish then add a telescopic eyed twin tailed Goldfish for variety and expect it to be able to compete for food, even though it can’t see the food properly nor can it swim as fast.

Goldfish sometimes have a deformity of the mouth opening that limits the amount of food they can get into their mouth, so they feed more slowly. These fish are always slower growing than their siblings.
If the mouth shape is the same as the other two fish, check that there isn’t something caught inside the mouth that is causing a blockage.

Are all your fish the same age and from the same spawn?
Older fish grow more slowly than younger ones. If the smaller fish was stunted at some stage, this will limit the maximum size it will attain.

There could be an internal problem with the intestinal tract, but unfortunately this isn’t a problem that can be easily fixed, if at all.


Is the smaller fish as active as the others?
Many diseases suppress appetite and activity.
Each Goldfish has a different immune capacity. It’s possible that your fish are carrying parasites that are affecting the smaller fish more acutely.

Most protozoan parasites such as Costia can be treated with a bath of 0.3% un-iodised salt for a week.

Flukes are a very small worm that can be treated with medications containing Praziquantel. It also kills internal worms and is harmless to fish plants and biological filters.

In order of importance I would check:

1. Water quality; ammonia zero, pH 7.0, nitrates no more than 20 ppm. This just ensures the fish are in the best possible water quality.
2. Observe the fish feeding to ensure enough food is being fed so all three get enough, and the smaller fish is actively eating. The smaller fish should have a rounded appearance and not appear thin compared to the other two fish. Is your food fresh and of high quality? He could be a fussy eater.
3. Check for physical issues with the mouth.
4. Treat for parasitical infestation.


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