Filter choice matters less than filter capacity and proper maintenance. A well-maintained $30 hang-on-back filter outperforms a neglected $300 canister. Most filter brand wars are theological — what matters is whether the unit moves enough water and provides enough surface area for beneficial bacteria. Below: what each filter type actually does, where each shines, and how to maintain any of them.
The three jobs of filtration
Every filter, regardless of type, does some combination of three things:
1. Mechanical filtration
Physically traps debris — uneaten food, fish waste, plant matter — in foam, floss, or sponge. Does nothing for ammonia or chemistry by itself; just keeps the water visually clear and prevents debris from rotting in the tank.
2. Biological filtration
The most important function. Provides surface area for the nitrifying bacteria that convert ammonia to nitrite to nitrate. Sponge, ceramic rings, bio-balls, and lava rock all work. A properly cycled biological filter is the difference between a healthy tank and a fish kill.
3. Chemical filtration
Removes dissolved substances through adsorption — activated carbon, GFO (granular ferric oxide), purigen, ion-exchange resins. Useful situationally, not always. Carbon removes some medications, tannins, and odors. Most established freshwater tanks don’t need it running constantly.
Filter types
Hang-on-back (HOB)
The default beginner filter. Hangs on the back of the tank, draws water up, runs it through media, returns it via a waterfall.
Best for
- 10-55 gallon tanks
- Beginner setups
- Anyone who values easy maintenance
Pros
- Cheap ($25-80 typical)
- Easy to maintain — open the lid, rinse media in tank water, close
- Returns water with gentle surface agitation (good for gas exchange)
- No tubing to clean or fail
Cons
- Limited media capacity
- Visible from the side
- Cannot be tuned for flow direction
Brands that hold up
AquaClear (Fluval) is the long-running benchmark — refillable media basket, reliable for 10+ years. Marineland Penguin uses a Bio-Wheel that adds biological surface area. Tidal/Seachem Tidal has good media volume for the price.
Canister filters
Sealed external canister with multiple media trays. Tubes draw water from the tank, push it through stacked layers of mechanical, biological, and chemical media, return it through spray bars or outflow nozzles.
Best for
- 40 gallons and larger
- Planted tanks (controlled flow, less surface disruption to preserve CO2)
- Community tanks where the filter shouldn’t be visible
Pros
- Large media capacity
- Silent (or near it)
- Adjustable flow direction with spray bars
- Can be hidden in a stand
Cons
- Expensive ($100-400)
- More involved maintenance — disconnect tubes, drain, clean
- Failure mode (a seal leak) can drain your tank onto your floor
Brands
Fluval (FX series for large tanks, 07 series for mid-size) — reliable mid-to-high-end. Eheim Classic — German engineering, decades of track record. Oase BioMaster — built-in pre-filter that simplifies maintenance. SunSun — budget option from China, surprisingly good for the price.
Sumps
A second smaller tank below or beside the main tank. Water gravity-drains down, gets processed through baffled chambers (mechanical, biological, sometimes a skimmer), and is pumped back up.
Best for
- Saltwater and reef tanks (effectively standard)
- Large freshwater tanks (75+ gallons)
- Anyone who wants to hide equipment (heaters, skimmers, ATO sensors) below the display
Pros
- Largest filtration capacity available
- Increases total water volume (more stable parameters)
- Equipment lives in the sump, not in the display
- Customizable to your needs
Cons
- Requires drilled tank or hang-on overflow
- Most expensive option
- Plumbing complexity
- Audible water-fall noise if not damped
Internal filters and sponge filters
Submerged inside the tank. Sponge filters powered by an air pump are the cheapest and most reliable biological filter on the market. Often used in fry tanks, hospital tanks, and shrimp tanks where the gentle flow won’t suck up small life.
Filter media basics
- Filter floss / poly fiber — fine mechanical, traps small particles, replaced when clogged
- Sponge / foam blocks — coarse to medium mechanical + biological; rinse in tank water, don’t replace
- Ceramic rings / Matrix / bio-balls — biological media. Rinse occasionally, don’t replace unless they crumble.
- Activated carbon — chemical; effective for about 4 weeks, then becomes biological media
- Purigen — chemical; removes organics; can be regenerated with bleach
The single rule of filter maintenance
Never rinse biological media in tap water. Chlorine and chloramine kill the bacteria you spent weeks growing. Rinse all media in old tank water during water changes. Replace floss-type media when clogged. Don’t replace biological media unless it physically breaks down.
Flow rate guidelines
- Freshwater community: 4-6x tank volume per hour
- Planted tank: 3-5x (lower for CO2-injected tanks)
- Cichlid / heavy bioload: 6-10x
- Saltwater fish-only: 5-10x
- Reef (sump + powerheads combined): 20-50x
These are starting points. Watch the fish — long-finned species hate strong flow; cichlids from rivers love it.
Common filter mistakes
- Replacing all media at once. Wipes out the bacterial colony. Replace only one type at a time, leave the rest alone.
- Cleaning too often. A filter only needs maintenance when flow drops noticeably. Monthly for most setups.
- Buying “replace the cartridge” filters and following the marketing. Replaceable cartridges are sized to fail — they need replacement before the bacteria mature. Buy filters with reusable media baskets.
- Undersized filter. A filter rated for your tank size is the minimum, not optimal. Going one size up provides margin.
Bottom line
For tanks under 55 gallons, an oversized AquaClear or Tidal HOB does 90% of what you need at a fraction of the cost of a canister. Above 55 gallons, a canister becomes the better choice. Above 100 gallons (or for any saltwater), consider a sump. Whichever you choose, leave the biological media alone, rinse only in tank water, and replace floss when it clogs. That is most of filtration maintenance.