Goldfish Care Tips

A Free Monthly Resource For Goldfish Enthusiasts
November 2025
Issue #143

In This Issue
Big Fish in Small Tanks

Goldfish can grow larger than most aquarium fish. With size comes problems: tank size, filtration, and what to do with your outsized fish.

Big Fish in Small Tanks

You see them all the time on Facebook. Someone posts an image of their aquarium with several large fish, and a tsunami of comments follows about how overcrowded or undersized the tank is.

The owner is subjected to a guilt trip because of the cruelty they are inflicting on their fish.

Funny thing is, the fish usually look healthy, and they appear completely unstressed by their environment.

How Much Room do Goldfish Really Need?

On my website, I have a page dedicated to answering this question, and I start by stating that there is no single correct answer. It’s all about maintaining water quality.

This has become more evident to me in the last year, because I have needed to grow small fish into large fish in small aquariums.

The image above shows a Black Moor I have raised from 30mm when I discovered her in a duckweed storage tub (I think it is a her, as she is less than a year old) to 100mm (nearly 4 inches) in a 60-liter (16 US gallons) aquarium with her two Water Bubble Eye companions.

Most of this growth occurred in 5 months before Winter’s cooler temperatures stopped growth.

How is this possible? The fish must be exhibiting symptoms of stunting, as discussed in last month’s E-Zine, or she must be displaying symptoms of disease.

As you can see, she is very chunky, very healthy, and very skilled at extracting food from her owner.

Dangers of Having Large Fish in Small Aquariums

I have proved that you don’t need a massive tank to grow Goldfish large, and by maintaining high water quality, their health isn’t affected.

I only change the water when the nitrates go over 40 ppm (parts per million), which occurs at around two weekly intervals, so no, I am not making daily 100% water changes.

Filtration

As shown in the image, I am running a sponge filter. I don't feel sponge filters are the best choice for large Goldfish in small aquariums because it is impossible to tell when they become clogged. The bubbles still rise to the surface, but that doesn't indicate water flow.

Canister or HOB (hang-on-back) filters are better, as you can see when the water flow decreases or bypasses the filter media.

Substrate

I don't use substrate in any of my tanks because of the additional work required for cleaning and the added bio-load from detritus buildup.

Equipment Failure

Goldfish need a dissolved oxygen level of at least 5 ppm. Recently, while away for a few days, the air supply to one of my tanks decreased significantly. When I arrived home, the two 90mm Moors were not happy and breathing heavily. If I had been away for another day or two, I would have lost them.

Conclusion

Can you grow Goldfish large in small aquariums? Yes, you can!

Are there downsides to this? Yes.

Water quality is harder to maintain, and if equipment fails or a prolonged power outage occurs, there is less safety margin before water conditions become toxic.

So, the rules given on my website page How Much Room do Goldfish Need? Still apply:

Get the biggest aquarium you can physically manage.
Install the appropriately sized filter.
Start with low numbers of small fish, and expect them to grow.
Buy a water test kit.

Comments? Ideas? Feedback? I'd love to hear from you. Just reply to this e-zine and tell me what you think, or what topics you want to be covered.

www.about-goldfish.com/

www.facebook.com/aboutgoldfish

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