20 Gallon Tank Overview
The 20 gallon aquarium comes in two standard configurations, each with distinct advantages. The 20 gallon High (24" × 12" × 16") has a smaller footprint but more vertical space, while the 20 gallon Long (30" × 12" × 12") provides more horizontal swimming room and bottom area. For most freshwater setups, the 20 Long is preferred — most fish swim horizontally, not vertically, and the extra length provides better territory distribution in community tanks.
At 20 gallons, you cross a threshold where community fishkeeping becomes genuinely viable. You can maintain stable schools of 10+ nano fish, keep species that need more space (like dwarf gouramis or a fancy goldfish), and create planted aquascapes with real depth and perspective.
20 High vs 20 Long
- 20 High (24" × 12" × 16"): Better for tall plants and aquascapes that use vertical space. Good for species that occupy different water levels. Takes up less counter/shelf space. Slightly harder to light evenly due to depth.
- 20 Long (30" × 12" × 12"): More swimming room for active fish. Better for bottom dwellers (more floor space). Easier to light, clean, and aquascape. Generally preferred by experienced aquarists. Excellent for breeding setups.
Best Fish for a 20 Gallon Tank
Community Tank (peaceful mix)
A classic community: 8–10 neon or cardinal tetras + 4–6 corydoras catfish + 1 dwarf gourami or pair of honey gouramis + 2–3 nerite snails. Covers all water levels — tetras in the mid/top, corys on the bottom, gourami as a centerpiece.
Livebearer Community
8–10 guppies or endlers + 6 platies or swordtails + 4 corydoras. Colorful, active, and prolific breeders (be prepared for babies).
Single Fancy Goldfish
20 gallons is the minimum for one fancy goldfish (fantail, oranda, ryukin). Add 10 gallons per additional goldfish. Not suitable for common/comet goldfish (need 30+ gal each).
Dwarf Cichlid Tank
A pair of German Blue Rams or Apistogramma with 8 tetras and 4 corydoras. More advanced but incredibly rewarding — dwarf cichlids display fascinating breeding behavior.
Planted Shrimp Tank
100+ cherry shrimp in a heavily planted aquascape with mosses, ferns, and crypts. Possibly the most relaxing setup in the hobby.
Essential Equipment
- Filter: HOB rated for 30–40 gallons (AquaClear 30 or 50, Seachem Tidal 35) provides adequate flow. A canister filter (Fluval 107) is a premium option that provides superior filtration and runs quieter.
- Heater: 100W adjustable is standard. In cooler rooms, consider 150W for extra capacity. Place near filter outflow for heat distribution.
- Light: For planted tanks, the Fluval Plant 3.0 (24"), Finnex Planted+ 24/7, or Nicrew SkyLED Plus are excellent options at different price points.
- Stand: At 225 lbs filled, a dedicated aquarium stand is strongly recommended. Standard furniture may not support this weight safely long-term.
20 Gallon Planted Tank
The 20 gallon — especially the Long — is considered one of the best sizes for planted aquascaping. It's large enough for dramatic layouts but small enough to keep costs reasonable. The 20 Long's shallow depth (12") makes it easy to light effectively without expensive fixtures.
A classic low-tech planted 20 Long might include: Vallisneria in the back, cryptocorynes and anubias in the midground, dwarf sagittaria or monte carlo in the foreground, and java moss on a piece of driftwood. No CO2 needed — just a decent light, root tabs, and liquid fertilizer.
Maintenance Schedule
- Weekly: 20–25% water change (~4–5 gallons). Gravel vacuum front sections. Test parameters if issues suspected.
- Bi-weekly: Rinse filter media. Trim plants. Clean glass.
- Monthly: Deep clean. Check equipment. Replace consumables.
Frequently Asked Questions
How many fish can live in a 20 gallon tank?
Roughly 15–20 small fish (under 2") or fewer larger fish. Example: 10 tetras + 6 corydoras + 1 centerpiece fish is a well-balanced community. Avoid overcrowding — it's the most common cause of water quality problems.
Should I get a 20 High or 20 Long?
20 Long is almost always the better choice for fishkeeping. More swimming space, more bottom area, easier to maintain and light. The only reason to choose High is if counter space is very limited.
Can I keep goldfish in a 20 gallon?
One fancy goldfish (fantail, oranda, ryukin) — yes. Common/comet goldfish — no, they need 30+ gallons each. Never keep two goldfish in a 20 gallon.
How much does a 20 gallon tank weigh?
Empty: ~25 lbs. Filled: ~225 lbs. A proper stand is highly recommended at this size.