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How Many Goldfish per Gallon for Aquaponics (Surprising Answer)

Lindsey Stanton Profile Picture

By Lindsey Stanton


Goldfish are a FANTASTIC fish for aquaponics. They provide lots of nutrients for your plants as well as being awesome, beautiful, fun-to-watch pets! But how many should you add in?

Is it something you can calculate on a per-gallon basis? Well, today, I’m going to give you the lowdown on how many goldfish to stock for aquaponics – and the answer might surprise you. Let’s dive in!


Stocking Rules: How Many Goldfish per Gallon for Aquaponics?

I get it: people want a straightforward answer when they ask this questionSomething easy to remember, like “one fish per gallon of water.” Or even “one pound of goldfish per cubic foot of grow bed.”

It really isn’t as simple as you might think. The number of goldfish you should use to stock your aquaponics tank is not a black-and-white answer because it really depends on so many variables – too many to be able to figure out in a broad brush one-liner.

Factors such as…

  • What kind of plants are you growing? Some plants are nutrient hogs, others aren’t.
  • How many plants do you have? More plants take up more waste.
  • How much media and what kind? Certain kinds of filter media are more efficient than others. How much you use also plays a huge role.
  • What food are you feeding, and how much at a time? Some low-quality foods foul the water more. If you feed your fish heavily, more waste is the result (I highly recommend a good quality aquaponic goldfish food).
  • What is your water change schedule? More water changes can allow for more fish.
  • What is your substrate? Some substrate gets dirtier faster than others, which can lead to ammonia problems if not cleaned enough with a heavy load of fish.
  • How big are your fish? Small fish produce less waste than larger ones as they require more food.

What Matters Most

The bottom line? Stocking density is not nearly as dependent on the gallons of water in the tank as it is the filtration capabilities of your system to process the nutrients.

It really comes down to water quality. Keep the water quality good, and you can support more fish than someone else with the same tank and number of fish.

Some people can maintain good water quality with 50 goldfish in a 50-gallon tank.


open mouth goldfish
Photo Credit: sarintra chimphoolsuk

Others struggle to keep their water acceptable with only 5 fish in the same volume of water. The reason is it has to do with all the variables above, and water volume may not be nearly as important as filtration capabilities since the per-gallon method is not a very accurate way to look at things.

So some recommend stocking 20-25 fish per 500L of grow bed while recognizing that influencing factors vary greatly. Whether you have too much fish or not enough fish depends on your unique system’s nutrient demands and usage.

So here’s the deal:

  1. If you find yourself without enough nutrients for your plants, you can add more fish and feed more (within reason).
  2. If you find yourself struggling to keep your water quality under control, you can add more filtration, cut down food, do more water changes, switch to a cleaner substrate, add more plants, and remove fish.

Some like to start with a smaller number of fish and add more if they feel it’s needed. Low and slow have benefits. This prevents having to rehome fish if you end up having too many if they grow up to be big (aerial plants remove growth-inhibiting hormones, so this is quite possible in a system).

It also gives your bacteria colony more time to adjust to the number of fish gradually. However, you run the risk of introducing disease if you don’t properly quarantine the new guys for 28 days minimum before adding them to your existing fish.

This means potentially having to go through multiple rounds of quarantine each time you get new fish (which can be a pain).

If you're terrified of passing on any bugs your new fish may have to your entire aquarium, or just want to be sure you do the quarantine process right, we recommend you read our best-selling book The Truth About Goldfish before you put them in.

The Truth About Goldfish New Edition

It has detailed instructions on a seamless quarantining process and so much more. Your fish will thank you!

Unless you get your fish from a reliable, clean source – though you would pay more for more shipments ordering online. The second option: you can start out with more fish, skip out on a separate quarantine tank and then modify your system to meet their needs if necessary as time goes on – I mentioned how to do this in point 2 above.

Stocking Relations to Goldfish Size

Another factor that gets brought up on the tank size topic is the size of the fish getting to be too much over time.

First, unless you plan on harvesting your goldfish to use as a food source, you probably don’t need to worry about trying to get them to grow huge.


Photo Credit: kaori, Pixabay

Second, in a more crowded environment without tons and tons of water changes, your goldfish probably won’t grow as large as they could otherwise. But chances are you don’t need them to anyway.

Most people start out with fingerling goldfish, those little 2-inch “feeders” sold at the pet store. Goldfish (like koi) can self-regulate their growth. Meaning that they can stay tiny when they don’t have tons of fresh water, and there is a lot of other goldfish in with them.

And I have yet to see any evidence that this has harmful effects on them. Now, if you want to grow yourself some monster goldfish and that’s your dream, more power to you.

If your fish aren’t very large to start with, stocking lightly might result in not enough nutrients for your plants in an aquaponics system until they grow large enough, which could take several years. (Not to mention, not all goldfish grow big no matter how much food, clean water, and space you give them.)

And as mentioned – too many and your filtration can max out, leading to problems like ammonia and nitrite spikes. It’s a delicate balance.

Water Changes Dependent on Stocking and Water Quality

In an aquaponics aquarium, you typically have one of the most – if not THE most – powerful filtration setups on the planet. So “overstocking” is usually not nearly as much of a problem as it could be in a regular aquarium setup (provided you cycle everything properly to start with, of course).

Actually, with aquaponic grow beds doubling as filtration, you can usually limit the water changes big time. That’s because the plants take out nitrate, which most regular filters don’t do.

Fewer water changes reduce the growth of the fish since you’re not removing the somatostatin that continually builds up in the water, thereby discouraging the fish from turning into huge monsters.

goldfish in tank with marbles substrate
Photo Credit: Pixabay


Maybe this post didn’t give you a hard-fast answer to your question, but I hope it helped to shed some light on how many goldfish to keep in your aquaponics system. What about you?

How many goldfish do you keep in your setup? I’d love to hear from you in the comments below.

Featured Image Credit:  josefkubes, Shutterstock

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