What fish can I put in a 20-gallon tank?

4 stocking themes work in a 20-gallon: Community freshwater, Nano reef (20g long), Single-species cichlid, Heavily planted Iwagumi.

How many fish for a 20-gallon tank?

Depends on species adult size. Use the stocking calculator at /calc/tank-stocking/ to verify your specific plan.

Best fish for 20-gallon beginner?

Stick to a planted community theme - cardinal tetras + corydoras + otocinclus + dwarf gourami pair. Hardy, peaceful, beginner-proof.

Can I have a saltwater 20-gallon tank?

Yes - 20-gallon supports saltwater FOWLR or nano reef. See the saltwater theme above for stocking.

Tank stocking ideas

20-gallon tank stocking ideas

4 themed stocking plans with exact species + counts + notes. Pick the theme that fits your goals, then verify with the stocking calculator.

4 themed stocking ideas for 20-gallon

1. Community freshwater

12x Cardinal tetras
6x Corydoras (sterbai or panda)
4x Otocinclus
1x Pair German blue rams
8-10x Amano shrimp

Notes: The all-time classic community. Schools up top, bottom dwellers + algae crew below, centerpiece pair in the middle.

2. Nano reef (20g long)

1x Pair Ocellaris clownfish
1x Yellow watchman goby + tiger pistol shrimp
20x Mixed CUC (turbo, nassarius, hermits)
Soft coral + LPS frags

Notes: 30-inch footprint allows aquascape depth. AIO setups (IM Nuvo Fusion 20) are the easiest path.

3. Single-species cichlid

6-8x Apistogramma cacatuoides (1M, 4-7F)
8x Corydoras habrosus
15x Cherry shrimp colony

Notes: Apisto harem with cory bottom-dweller. Highly territorial but interesting breeding behavior.

4. Heavily planted Iwagumi

20x Chili rasbora school
10x Otocinclus
30x Cherry shrimp colony
Carpet (Monte Carlo or HC) + Iwagumi stones

Notes: Nature Aquarium style. High light + CO2 + EI fertilization. Highest visual impact for a nano.

Verify your stocking plan

Use the stocking calculator to verify the exact species + counts work in your 20-gallon. Also check the water change calculator for ongoing maintenance, and the setup guide library for the full build playbook.

Other tank sizes

Browse all stocking idea pages from 5g nano to 180g show-tier. Or jump to the stocking blueprints for build-along plans with timelines.

Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.