How Many Fish Can Go in a Tank?
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Overstocking is easy to do. Here's what the guidelines say, and where they fall short.
The rule
CDC: "a good standard is 1 inch of fish per gallon of water." Purdue Extension: "one 1 inch fish (not counting the tail) for each gallon of water." An older Purdue 4-H manual adds a key reminder: "It is best to allow fish plenty of room until you have gained experience, because overcrowding is usually detrimental. Remember: fish grow and multiply!"
Where the rule breaks down
We didn't find an extension source that spells out the rule's limits, but the same sources point to them:
- Adult size. Habitattitude says to ask: "What is its adult size? Will it outgrow the tank?" Minnesota DNR says goldfish typically reach 5 to 8 inches and can reach close to 2 feet.
- Waste. Aqueon says goldfish "generate a significant amount of waste," and recommends at least 20 gallons per adult common goldfish, far more than an inch per gallon.
- Temperament. Habitattitude asks: "Is this a peaceful species? Is it compatible with other tank mates?" Aqueon says male bettas must be kept apart.
Stock slowly
Aqueon: "Adding too many fish at once or overfeeding a new freshwater aquarium will cause ammonia and nitrite to rise to unsafe levels – a condition known as 'New Tank Syndrome.'" Aqueon's advice is to "test water regularly until ammonia and nitrite levels stabilize at zero" before adding more. Read how to cycle a fish tank first.
Feed lightly
UF calls overfeeding "a major cause of high ammonia concentrations." Purdue's 4-H manual says to feed only what fish "clean up ... in five minutes"; Aqueon says 2 to 3 minutes for goldfish.
Bigger tanks help
Habitattitude says larger tanks are less likely to swing in temperature, oxygen or toxic compounds. See starter kits.
Tetra 20 Gallon Aquarium Kit
- 24 x 12 x 16 in. glass tank with hinged LED hood, per the listing
- Internal filter with carbon cartridge
- Heater and thermometer included
Tetra 29 Gallon Aquarium Kit
- 30 x 12 x 18 in. glass tank, LED hood, per the listing
- Whisper PF 30 filter
- 200W heater and thermometer included
API Freshwater Master Test Kit
- Tests pH, high-range pH, ammonia, nitrite and nitrate, per the listing
- 7 test solutions, color card and 4 tubes
- Listed as 800 tests; freshwater only
FAQ
Inch-per-gallon?
About 1 inch of fish per gallon (CDC, Purdue).
Current or adult size?
Adult size.
All at once?
No; stock gradually.