Equipment sizing
Aquarium filter size guide
Buy for adult waste and how you clean media — not the litre sticker on the glass. Most “failed” filters were rinsed under the tap, clogged at the impeller, or never matched the bioload.
What a filter is doing
Three jobs. Mechanical catches debris. Biological surface grows the bacteria that turn ammonia into nitrite, then nitrate. Chemical media (carbon, resins) is optional — it does not replace water changes or a working cycle.
Flow moves water. Media volume grows the colony. You need both.
Turnover bands by tank size
Keepers talk about turning the tank over several times per hour. Treat that as a starting band. Box L/h is usually measured with almost no media in the way, so a packed canister or hang-on-back pushes less than the label. Goldfish and heavy feeders need more capacity. Bettas and discus need the same biology with a softer return — spray bar, baffle, or sponge prefilter — not a dead filter.
Light column ≈ 4–6× rated turnover. Heavy ≈ 8–10×. Crowded tanks may still need more media at the same L/h. Fish behaviour and climbing nitrate beat any table.
| Tank | Light / modest stock | Heavy / messy stock |
|---|---|---|
| 30L · 8 US gal | 120–180 L/h | 240–300 L/h |
| 40L · 11 US gal | 160–240 L/h | 320–400 L/h |
| 60L · 16 US gal | 240–360 L/h | 480–600 L/h |
| 80L · 21 US gal | 320–480 L/h | 640–800 L/h |
| 100L · 26 US gal | 400–600 L/h | 800–1000 L/h |
| 120L · 32 US gal | 480–720 L/h | 960–1200 L/h |
| 200L · 53 US gal | 800–1200 L/h | 1600–2000 L/h |
Stocking, room temperature, tank shape, and how often you clean all move the answer. The tables here are shortlist bands — check them with a probe thermometer, your eyes on flow, and liquid tests.
Light vs heavy midpoints
Same bands as the table, midpoints only — for a quick glance.
When higher flow is the wrong answer
- Long-finned fish pinned against décor.
- Surface so choppy that labyrinth fish avoid the top.
- Planted tanks where fine leaves get blasted (soften the return; do not gut biology).
- A nano with a “rated for 200L” powerhead turning the tank into a whirlpool.
Waste still has to be processed. Add media, soften the return, or run a second gentle filter. Do not delete filtration.
How bioload changes the answer
Two 60L tanks are not the same job. A lightly fed tetra school with weekly water changes is not three goldfish or a messy digger. Adult size, feeding, and how often you vacuum beat the litre print on the glass.
Check the real list in the tank builder. Crowded or protein-heavy plans → heavier column. Slow-water species → gentler return even if the rating looks big enough.
Filter types compared
Sponge
Air-driven or small powerhead. Shrimp- and fry-safe, cheap, excellent as a bacteria farm. Weak as sole filtration for heavy adult bioload.
Best for: shrimp, fry, quarantine, gentle nanos.
Internal / power filter
Compact and inexpensive. Limited media volume. Fine for light loads and hospital tanks; outgrown quickly by busy communities.
Best for: <40L, quarantine, backup.
Hang-on-back (HOB)
Easy media access, good oxygenation from the waterfall. Splash and noise vary. Strong option for many 40–200L communities when cleaned on schedule.
Best for: beginner community tanks.
External / canister
Large media volume, quiet plumbing, flexible returns. Higher cost and cleaning discipline. Suited to larger or heavily stocked systems where biology needs room.
Best for: 100L+, planted, higher bioload.
Common filter mistakes
- Buying by the tank label only. “For 60L” assumes a light stock and clean media. Adult goldfish or large cichlids rewrite that label.
- Confusing flow with biological capacity. A skinny high-flow unit can still be media-poor.
- Cleaning media under untreated tap water. Chlorine and chloramine kill the colony. Rinse in old tank water.
- Replacing all media at once. Stagger changes over weeks.
- Ignoring the impeller. Flow dies quietly when the impeller is packed with gunk.
Size the filter to a real stock list
Pick a flow band from the table, then check the real stock list in the tank builder.
Also see the heater size guide, tank setup checklist, and volume hubs: 30L · 60L · 100L · 200L.
Sources and limits
Stocking, room temperature, tank shape, and how often you clean all move the answer. The tables here are shortlist bands — check them with a probe thermometer, your eyes on flow, and liquid tests.
- Aquarium Co-Op — The Nitrogen Cycle — Clear ammonia → nitrite → nitrate walkthrough for cycling before livestock.
- Seriously Fish — species care profiles — Temperature and flow notes when you leave the generic table for a real stock list.
Filter sizing — quick answers
- What size filter do I need for my aquarium?
- Start with volume, then add for waste. Light column ≈ 4–6× rated turnover; heavy ≈ 8–10×. Messy tanks often need more media, not just a higher L/h sticker. Bettas and other slow swimmers need the return softened, not biology removed.
- Is 5× or 10× turnover a hard rule?
- No. Box ratings are usually measured with almost no media in the way. Packed canisters and hang-on-backs push less water than the label. Clean media beats chasing a magic multiplier.
- Sponge vs hang-on-back vs canister — which is best?
- Depends on the tank. Sponge for fry, shrimp, quarantine. Hang-on-back for many beginner communities. Canister when you need media room on a bigger or messier system. Pick what you will actually clean.
- Why did my cycle crash after filter cleaning?
- The bacteria live on wet media. Tap water with chlorine, or swapping every cartridge at once, wipes them. Rinse in old tank water. Stagger media changes.