Is a 20 high or 20 long harder to filter?

A 20 long is easier to filter and circulate because it has more surface area and a shorter water column, so flow spreads evenly. A 20 high has a taller column where the bottom can go stagnant, so aim a hang-on-back return or a powerhead to push flow downward.

Best filter for a 20 gallon tank

Q: What size filter do I need for a 20 gallon tank?

Aim for about 100 to 150 GPH of real flow on a 20 gallon tank, which is 4 to 6 times the volume per hour. A hang-on-back rated around 200 GPH, such as a Seachem Tidal 55 or AquaClear 50, hits that with headroom once you account for media and lift.

Q: What is the best filter for a planted 20 gallon tank?

For a planted 20 gallon, a small canister like the Fluval 107 is the best choice: it hides the equipment, returns gentle, CO2-friendly flow through a spray bar, and holds more media than any hang-on-back. A baffled AquaClear 50 also works if you prefer to stay with a HOB.

Short answer: a 20 gallon tank wants about 100–150 GPH of real flow. The best all-round pick is a Seachem Tidal 55 or AquaClear 50 hang-on-back. For a planted 20, a small Fluval 107 canister is the upgrade. The thing to get right at this size is flow distribution — a 20-high behaves very differently from a 20-long.

The pick, in one line each

Best overall — Seachem Tidal 55 (HOB). Rated ~250 GPH, self-priming, surface-skimming intake, and a big media basket you can pack with whatever you like. The 20 gallon sweet spot.
Also excellent — AquaClear 50 (HOB). The classic. Adjustable flow, bulletproof, endlessly customizable media. Tidal vs AquaClear is preference; both are right.
Budget alternative — Aqueon QuietFlow 20. Quiet and cheap for a basic community 20; cartridge-based, so less media flexibility.
Best for planted — Fluval 107 canister. Hides the gear, gentle spray-bar return that won’t gas off CO₂, and far more media than any HOB.

Why 100–150 GPH

Four to six tank turnovers an hour keeps a 20 gallon clear and well-oxygenated and gives the beneficial bacteria enough water passing through their media. That is 100–150 GPH of real flow. Rated GPH is measured empty with no lift, so you lose 30–50% in practice — buy a filter rated around 200 GPH to land on target. Keeping goldfish or a heavily-stocked community? Push toward the 6–10× range (up to ~200 GPH) and lean to the canister.

20 high vs 20 long — the detail most pages skip

“20 gallon” is two very different tanks. A 20 long (30″ × 12″ × 12″) has lots of surface area and a short water column, so a single HOB circulates it easily and gas exchange is excellent. A 20 high (24″ × 12″ × 16″) has a tall column where the bottom corners can go stagnant. On a 20-high, aim the HOB return to push flow downward, or add a small powerhead, so the whole tank turns over rather than just the top third.

Recommended gear · affiliate
Filters for a 20 gallon tank. As an Amazon Associate, TankStocked may earn from qualifying purchases — at no extra cost to you.

Seachem Tidal 55 (best overall) · AquaClear 50 (classic HOB) · Aqueon QuietFlow 20 (budget) · Fluval 107 canister (planted)

GPH targets are a starting point, not a law. Lightly stocked or planted tanks do well with less flow; goldfish and cichlids want more. Model suggestions reflect each maker’s published tank-size rating and common hobbyist consensus, not a guarantee of fit for every setup.

Frequently asked questions

What size filter do I need for a 20 gallon tank?

Target 100–150 GPH of real flow. A HOB rated around 200 GPH — Seachem Tidal 55 or AquaClear 50 — covers it with headroom.

What is the best filter for a planted 20 gallon tank?

A small canister like the Fluval 107: it hides the equipment, returns gentle CO₂-friendly flow, and holds more media than any hang-on-back.

Can I keep goldfish in a 20 with a 20-gallon filter?

Goldfish are messy, so size up: target the 6–10× range (toward 200 GPH) and consider a canister. A 20 is also only suitable for one fancy goldfish long-term — check the stocking guide first.

Sizes either side: 10 gallon · 30 gallon · Full filter size chart · Filter calculator

Affiliate disclosure