Complete 20-gallon aquarium setup guide. Equipment list, cost breakdown, stocking ideas, lighting + filtration sizing, and the best fish + invertebrates for a 20-gallon tank.
The 20-gallon footprint is the sweet spot where parameters stop swinging wildly and stocking options expand dramatically. At this volume, water-quality swings are slower than smaller tanks, which makes it more forgiving for new keepers and more rewarding for experienced ones because you can finally stock the species you actually want.
A 20-gallon aquarium full of water + sand + rock weighs roughly 210 pounds. Most furniture-grade stands handle this load fine, but level the stand before filling - 1/8" out-of-level over the long axis is enough to seam-stress a glass tank over time.
For a 20-gallon system you need: a heater (100W), an LED light (Glass canopy + clip-on LED), a HOB or sponge filter rated for 40+ gph, dechlorinator, a refractometer (saltwater) or GH/KH test kit (planted), a thermometer, and a basic test kit (ammonia/nitrite/nitrate). Mid-line, expect $200-450 for the full equipment stack before livestock.
Lighting depends on what you're keeping. A clip-on LED with adjustable brightness is enough for a planted nano or low-light freshwater. For a nano reef, step up to a Kessil A80, AI Prime 16HD, or equivalent. Filtration should turn over the display volume 4-6x per hour minimum. If you're running a HOB or canister, oversize it - filter ratings are typically generous, and an "20 gallon" canister filter actually performs best when paired with a tank one size smaller.
a school of 8-10 small tetras + corydoras; a pair of dwarf cichlids; a 20H reef with a single small fish + 6-8 frags. The honest mistake most aquarists make at this size is overstocking based on rule-of-thumb counts ("1 inch per gallon") that ignore territorial behavior, adult size, and bioload. Real-world stocking is determined by the species' adult footprint + temperament. Browse freshwater livestock, saltwater fish, and coral filtered to your tank size on Fast Aquatics, where every listing shows minimum tank size called out by the vendor and verified against the species record.
Real-world all-in costs for a working 20-gallon system land in this range: $300-650 (hardware + first stocking). Vendors on Fast Aquatics ship overnight via FedEx Priority and UPS Next Day with carrier-tracked Buyer Protection on every order, so you can mix charters from multiple specialists in one cart and have them all routed climate-aware to your door.
A pair of dwarf cichlids; a small tetra school of 6-8; a single discus (not recommended - they're shoaling); a clownfish pair with no other fish.
No - canister or HOB filtration is fine and a sump adds plumbing complexity that is only worth it for reef tanks.
Overstocking. The "1 inch per gallon" rule produces tanks that crash on minor parameter swings. Cut your initial stocking plan by half and add slowly over 6-12 weeks.
Yes - this is a great first-tank size. Big enough to be parameter-stable, small enough to be affordable and easy to maintain.