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Swimming FishiesFreshwater & saltwater care

Stocking calculator

Enter the tank you actually have. We suggest conservative communities from the species we write about — adult size, schooling needs, and waste, not a ruler.

This is a starting point, not a guarantee. Cycle the tank before you add fish. Plan for adults, not the juveniles on the store rack.

Your tank

Rated gallons, not the water line after gravel. Results update as you change the fields.

Water
Experience

20 gal · Freshwater · New to the hobby

Example communities

Pick one idea. Do not stack every species that could theoretically survive. Counts assume adult fish in a cycled tank.

  • Betta centerpiece

    One male in a planted 20 is easier to keep stable than a 5. Do not add a school unless he ignores other fish.

    • 1 × Betta (Betta splendens)One male. Heated, filtered, not a vase. Two males is a fight.

  • Neon and peppered cory community

    Midwater school plus a bottom group. That is a common, conservative 20-gallon community — not inch-per-gallon math.

    • 13 × Neon tetra (Paracheirodon innesi)A school, not a pair. Dimmer light and stable, clean water.
    • 6 × Peppered cory (Corydoras paleatus)Bottom school on sand. Six is a starting group, not a display pair.

  • Male guppies with cories

    Livebearers up top, a cory group on sand. Keep the guppies male-only so the bioload does not explode.

    • 5 males × Guppy (Poecilia reticulata)Males only unless you want fry and a ballooning bioload.
    • 6 × Peppered cory (Corydoras paleatus)Bottom school on sand. Six is a starting group, not a display pair.

    Mixed-sex guppies will outpace this plan.

Skip for this tank

  • Goldfish — Adults want about 30 gallons, not 20. Intermediate care. Cycle first and get comfortable testing water.
  • Angelfish — Adults want about 30 gallons, not 20. Intermediate care. Cycle first and get comfortable testing water.
  • Oscar — Adults want about 75 gallons, not 20. Advanced husbandry — wait until you are ready for the mess.

Gear that belongs with this

Filter, heater, and a real test kit. Links go to notes on this site — not a shopping cart.

  • Test kit

    Aquarium water test kit

    Liquid tests for ammonia, nitrite, nitrate, and pH. Cycle the tank before you add livestock.

  • Filter & heater

    20-gallon aquarium starter kit

    A 20-gallon kit should include a filter, a heater, and a lid. Many do not. Read what to look for before you buy the box.

Still cycling? How to cycle a fish tank · Beginner freshwater setup · Stocking a 20-gallon tank

Frequently asked questions

Is this the inch-per-gallon rule?+

No. Inch-per-gallon treats a 10-inch oscar like ten 1-inch tetras. This tool uses adult tank minima, schooling needs, and a conservative waste budget from the species we actually write about.

Why does a 5-gallon only get a betta?+

A 5-gallon is a minimum for one male betta, not a community tank. Schools and bottom dwellers need more footprint than that.

Can I mix every species that fits?+

Usually no. Goldfish are coldwater. Oscars eat tank mates. Male bettas may hunt. Pick one example community, not a shopping list of everything that could theoretically survive.

Do I still need to cycle?+

Yes. The calculator assumes a cycled, filtered tank and adult fish. Read the cycling guide before you buy livestock.