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The Goldfish Gazette, Issue #066 -- Aquarium Size
June 30, 2019

Goldfish Care Tips

A Free Monthly Resource For Goldfish Enthusiasts
June 2019
Issue #066

In This Issue
Aquarium Size - How Big is Big Enough?

One of the most important decisions to make is how big an aquarium to buy for your Goldfish.

Aquarium Size - how big is big enough?

When aquarium size requirements are discussed on websites, in publications or on social media, there is lots of advice given about the number of gallons each Goldfish needs, mainly based on whether the fish is a fancy variety or a single tailed variety.

The advice usually given is single tailed varieties need more gallons per fish because they are more active, and grow larger.

Perhaps, but a 6 inch (150mm), deep bodied Oranda can have the same body weight as a 12 inch (300mm) Comet. It will consume the same amount of food and produce the same amount of waste.

What I am trying to show here is, most of the advice given is flawed, because the advice isn’t backed up with more specific information.

The size of the aquarium you need is not the most important consideration.

The most important consideration is…how will you maintain high water quality?

Maintaining High Water Quality

We know for Goldfish to thrive, their water must have a good oxygen level, zero ammonia, nitrates below 30 ppm (parts per million) and ideally a pH around 7.40.

An aquarium is a closed water system, unlike a stream that gets flushed constantly. This means as soon as fresh water in an aquarium is populated with fish, it starts to become polluted.

How quickly water becomes polluted is determined by these factors:

1. The number and/or size of fish per gallon/liter
2. The food being fed and how much (high protein foods pollute more than vegetable-based foods)
3. How much plant life is present
4. Whether filtration is installed.

Even with a filter installed, water quality still drops; it just takes longer before the water quality drops to a level that harms fish. Filters do take away the worry of ammonia build-up but this process creates nitrates which filters can’t eliminate.

Water Changes Are Key to High Water Quality

Unless you will be setting up a balanced aquarium, which is one that can maintain pristine water conditions due to very low numbers of fish, large quantities of water plants in a very large aquarium, water changes will be the way most of us maintain high water quality for our fish.

Water changes are another piece of advice that varies depending on where the advice is coming from.

The amount to change can vary from 10-30% weekly, fortnightly or monthly, with no reference to fish population levels or whether filtration is installed. In other words, they are guessing.

So how is all this helping decide the size of aquarium we need?

What we do know is the water conditions we need to maintain for our fish to thrive.

We also know that water starts to become polluted as soon as fish populate it, and things such as fish numbers, food, plant life, filters and aquarium size all have an effect on how quickly that water quality drops to a dangerous level.

What to Really Consider When buying an Aquarium for Goldfish

We would all like to buy a 200 gallon aquarium for our Goldfish, but when we may only have two or three 2 inch (52mm) Comets, do we really need to start out that large?

Here is a list of some of the questions you need to answer before buying an aquarium and fish:

• How much physical space do I have?
• How much time do I have available to maintain the aquarium?
• How easy will it be to do water changes?
• How many fish, and of what variety do I want to keep?
• Do I want the fish to grow?
• Is there a power supply close by?
• Will a filter system be installed?

The answers to these questions then determine how easy or hard it will be to maintain high water quality for your Goldfish.

As examples, a 15 gallon aquarium with 2 three inch (75mm) Comets and a filter will require a complete water change fortnightly.

A 15 gallon aquarium with 4 three inch Comets and no filter will need a complete water change twice weekly.

Of course, I am only guessing, because I don’t know if you will have water plants, and how heavily you will feed your fish.

So how does the Goldfish owner know when, and how much water needs to be changed?

By using a water test kit.

A water test kit takes the guessing away. It will show when ammonia is present (if you don’t have a filter), and when the pH is starting to drop.

If you find you can’t keep up with the number of water changes required to maintain high water quality, then there is a conflict between one or more of the answers to the questions above, for example you have space for a 20 gallon aquarium, you want to keep 5 three inch fish but you can’t run a filter.


  • Get the biggest aquarium you can physically manage.
  • Install the appropriately sized filter.
  • Start with low numbers of small young fish.
  • Buy a water test kit.
Use the water test kit to establish how often you need to make water changes, and if it is too often, reduce the number of fish, or get a larger aquarium.

If you start with a small aquarium, and maintain pristine water conditions for your fish, they will grow so at some stage you will need to up-size, or reduce fish numbers.

For more detailed information about how much room Goldfish need, click here…

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 covered.

Next Month's Topic

Unplanned Spawnings

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