Goldfish are popular, beautiful fish, but they need more space than many beginners realize.
Unlike bettas or angelfish, goldfish produce a large amount of waste, so proper tank size and stocking are critical for their health and longevity.
Understanding Goldfish Needs
-
Goldfish are messy eaters and create high bioload.
-
They need powerful filtration and frequent water changes.
-
Most goldfish varieties grow much larger than people expect.
-
Adequate swimming space and oxygenation are essential.
How Many Goldfish in a 20-Gallon Tank?
-
Common Goldfish (Single-tailed): Not suitable – they grow up to 12+ inches and require at least 75 gallons for one fish.
-
Fancy Goldfish (Double-tailed varieties like Oranda, Ryukin, Fantail):
-
You can keep 1 fancy goldfish comfortably in a 20-gallon tank.
-
At most, 2 small juvenile fancies, but they will quickly outgrow the space and need a larger tank.
-
Why This Stocking Works
-
Goldfish produce heavy waste; overcrowding quickly pollutes water.
-
A single fancy goldfish can thrive in 20 gallons with proper filtration.
-
Common goldfish are too large and active for this tank size.
Tank Setup Tips for a 20-Gallon Goldfish Tank
Filtration & Water Quality
-
Use a powerful canister or HOB filter rated for a 40+ gallon tank.
-
Do 30–40% weekly water changes.
-
Test water regularly for ammonia, nitrite, and nitrate.
Heating & Temperature
-
Goldfish prefer cooler water: 65–75°F.
-
Heaters are usually not required unless your home is very cold.
Aquascaping & Enrichment
-
Avoid sharp decorations that could tear fins.
-
Provide smooth rocks, driftwood, and hardy plants like Anubias or Java fern.
-
Leave open swimming space since goldfish are active.
Tankmates for Goldfish in a 20-Gallon Tank
-
Best kept alone due to space limitations.
-
If upgrading later, suitable tankmates include white cloud minnows, dojo loaches, or other fancies.
Pros of a 20-Gallon Goldfish Tank
-
Great beginner size for one fancy goldfish.
-
Easier to maintain than a bowl or smaller tank.
-
Allows aquascaping and plant growth.
Cons of a 20-Gallon Goldfish Tank
-
Too small for common goldfish.
-
Can only house one fancy long-term.
-
Requires frequent cleaning due to waste.
Conclusion
A 20-gallon tank is suitable for 1 fancy goldfish only.
While you may temporarily keep 2 juveniles, they will soon need a larger tank.
Common goldfish should never be kept in a 20-gallon setup, as they grow far too large.
If you want more than one goldfish, plan for a 40–55 gallon tank or bigger.
FAQs
1. Can I keep 2 goldfish in a 20-gallon tank?
Only as small juveniles—eventually, they’ll need more space.
2. Is a 20-gallon tank enough for a common goldfish?
No, common goldfish need 75+ gallons.
3. How many fancy goldfish in 20 gallons?
Just one adult fancy goldfish.
4. Do goldfish need a filter in a 20-gallon tank?
Yes, a strong filter is essential.
5. Can I add other fish with my goldfish in 20 gallons?
Not recommended—stick with one fancy goldfish.