Goldfish Tank Calculator
Minimum goldfish tank gallons and weekly changes.
What it does
Goldfish outgrow bowls by an order of magnitude. One fancy goldfish needs 20 gallons minimum with 10 per extra, so three need a 40 gallon tank with 151 liters and 10 gallon weekly changes. Small bowls stunt growth and poison water within days.
The tool above sizes US tanks to fish counts. Enter fancy goldfish and it prints minimum gallons, liters, and the 25 % weekly change with a 5x filter rule attached. Defaults show the trio at 40 gallons; four need 50 with 12.5 gallon changes.
Welfare sources converge from both sides of the Atlantic. RSPCA requires 60 liters each, OATA asks 100 liters plus 50 per extra adult, and PetMD vets quote about 20 gallons per fish with 5-times hourly filtration. The 20-plus-10 rule sits practically between them.
Filters and changes matter as much as volume. Cycle tanks before stocking, change a quarter weekly with dechlorinated water, and test ammonia and nitrite to zero. Single-tail commons and comets belong in ponds at any count.
How to use
- Enter the fancy goldfish count.
- Read minimum gallons first, then liters.
- Read the weekly 25 % change.
- Size filters at 5x hourly turnover.
- Cycle the tank before stocking.
- House single-tails in ponds.
Formula
- fish
- Fancy goldfish count, 1 to 100.
US practical default between RSPCA (60 L each) and OATA (100 L + 50 each); PetMD vets quote ~20 gal per fish with 5x filtration.
Worked example
Example: 3 goldfish
Defaults: 3 fancy goldfish.
- Input: 3 fancy goldfish.
- Gallons: 20 + 10 x 2 = 40.
- Liters: 40 x 3.78541 = 151.4.
- Weekly change: 40 x 25 % = 10 gal.
- Filter 5x hourly. Tool shows 40 gal minimum, 151.4 L, 10 gal change.
Reference table
| Fish | Gallons | Weekly 25% |
|---|---|---|
| 1 | 20 | 5.0 |
| 2 | 30 | 7.5 |
| 3 | 40 | 10.0 |
| 4 | 50 | 12.5 |
| 5 | 60 | 15.0 |
Limits
- Fancy goldfish only. Single-tails need ponds.
- Minimums, not ideals; bigger always better.
- Ignores stocking of tankmates.
- Water chemistry needs test kits.
Common mistakes
- Bowls for any goldfish.
- Counting single-tails by fancy rules.
- Skipping the nitrogen cycle.
- Weak filters under 5x turnover.
- Missing weekly changes.
Try it yourself
Quick checks. Answers sit tucked under each question.
1 goldfish?
20 + 10 x 0 = 20 gal. Liters 75.7. Change 5 gal.
4 goldfish?
20 + 10 x 3 = 50 gal. Liters 189.3. Change 12.5 gal.
Questions
How many gallons per goldfish?
20 gallons for the first plus 10 per extra, for fancy goldfish. Three need 40 gal.
How many liters?
Gallons times 3.78541. Forty gallons is 151.4 L.
How much water changes weekly?
25 % weekly: 10 gal on a 40 gal tank. RSPCA and OATA agree on weekly partials.
What filter turnover?
At least 5 times tank volume hourly: 200 gph on a 40 gal tank, per PetMD.
Do UK rules differ?
Yes, 60 L each per RSPCA and 100 L plus 50 per extra per OATA. The 20+10 US rule sits between.
What about common goldfish?
Yes. Single-tail pond fish belong in ponds, not tanks, at any count.
Sources
- RSPCA: Choosing an Aquarium (60 L each)
- OATA: Goldfish Aquarium Care (100 L + 50 each)
- PetMD: Goldfish Care (~20 gal, 5x filter)