20 gallon fish tank: 6 stocking ideas

Stocking ideas · About 6 minutes to read

A 20 gallon tank is the sweet spot for a first community tank. It's big enough for a real mix of small fish, and stable enough to forgive mistakes. Here are six setups that work, each checked with the free stocking calculator.

20 high or 20 long?

A 20 high is about 24 inches long and 16 inches tall. A 20 long is about 30 inches long and 12 inches tall. Both hold about the same water, but the long version gives active fish more swimming room and bottom fish more floor. If you can choose, pick the long.

Before you buy fish

Cycle the tank first. See how to cycle a new fish tank.
Add fish in stages. Start with the hardiest group, then add the rest a week or two apart.
Quarantine new fish if you can. See how to set up a quarantine tank.

Six setups that work

Each plan below opens in the calculator with one click, so you can change the numbers and see the result.

1. Classic community

6 harlequin rasboras · 1 honey gourami · 6 pygmy corydoras

Keep at 72–79°F · 76% stocked in the calculator
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Peaceful fish at every level of the tank. The honey gourami is a calm centerpiece, and the harlequins school together in the middle.

2. Betta community

1 male betta · 8 ember tetras · 6 pygmy corydoras

Keep at 76–79°F · 50% stocked in the calculator
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Lightly stocked and very forgiving. Add the betta last, and keep an eye on him for the first few days.

3. Livebearer tank

5 platies · 6 pygmy corydoras

Keep at 72–79°F · 81% stocked in the calculator
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Platies are hardy and come in lots of colors. They breed easily, so choose all males, or have plants for the babies and a plan for extras.

4. Fast and busy

6 zebra danios · 6 pygmy corydoras · 1 nerite snail

Keep at 72–77°F · 68% stocked in the calculator
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Danios zip around all day. They do best in a "20 long" tank, which has more swimming room. Keep a lid on, because they jump.

5. Cherry barbs

6 cherry barbs · 1 honey gourami · 6 pygmy corydoras

Keep at 73–79°F · 76% stocked in the calculator
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Cherry barbs are a peaceful barb, with bright red males. Keep more females than males to keep things calm.

6. Corydoras tank

6 bronze or peppered corydoras · 1 honey gourami

Keep at 72–78°F · 85% stocked in the calculator
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For people who love corydoras. Bigger corydoras make more waste, so this plan is already near the limit. Use sand or smooth gravel, and feed sinking wafers.

Fish to avoid in a 20 gallon

Goldfish. They need cooler water than tropical fish, and fancy goldfish need 30 gallons or more.
Angelfish. They grow tall and eat small fish like neons and embers. Plan on 30 gallons or more.
Mollies, swordtails and pearl gouramis. They grow too big for a 20. Plan on 30 gallons.
Tiger barbs. Fin nippers that need a big group and a 30 gallon tank.
Common plecos. They can grow over a foot long. Choose a bristlenose pleco instead.

Gear for a 20 gallon tank

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Want more ready-made plans? Your First Aquarium includes stocking plans for 10, 20 and 29 gallon tanks, plus care cards for 15 beginner fish. Get the guide for $12