A filter does three jobs: it traps solid waste, it gives nitrifying bacteria a place to live, and (optionally) it adsorbs dissolved compounds. This guide explains how each stage works, compares the six filter types you will actually meet, and shows how to size flow with a worked example so you can pick a filter that suits your tank, your fish and your power supply.
Use the Filter GPH Calculator alongside this guide: enter your tank volume and stocking style, and it returns a target flow range you can compare against the numbers below.
What does an aquarium filter do?
A filter pulls water through media, returns it to the tank and, in doing so, keeps waste from piling up. It does not remove nitrate; water changes and fast-growing plants do that. Most filters combine the three stages below, in this order.
Mechanical filtration
Mechanical filtration physically traps solids such as uneaten food, fish waste and plant debris before they rot into ammonia. Typical media are coarse sponge, fine sponge, filter floss and polishing pads. Water should meet the coarsest media first and the finest last, so the fine media does not clog in days.
Mechanical media is the part that needs regular attention: it fills up, flow drops, and trapped debris starts to decay. Rinse it every 2-4 weeks in a bucket of old tank water, not tap water.
Biological filtration
Biological filtration is the stage that keeps fish alive. Ammonia-oxidising bacteria (such as Nitrosomonas) convert ammonia to nitrite, and nitrite-oxidising bacteria (such as Nitrospira) convert nitrite to nitrate. These bacteria grow on any surface with steady oxygenated flow: ceramic rings, sintered glass, lava rock, bio-balls and the walls of a sponge.
How much media you need depends on the bioload. A common planning figure for moderate stocking is roughly 1-2 litres of bio-media per 50 US gallons (about 190 L), and more for goldfish, cichlids or heavily stocked tanks. Treat that as a starting point, not a rule. Media that is clogged or starved of flow supports far fewer bacteria than clean media with good flow through it. See the nitrogen cycle guide for how the colony establishes.
Chemical filtration
Chemical filtration removes dissolved substances rather than particles. Activated carbon adsorbs odours, tannins that tint the water yellow, and many medications. It saturates, so it is normally replaced every 3-4 weeks when in use. Other options include Purigen (a regenerable organics remover) and zeolite (an ammonia adsorbent, useful only as an emergency stopgap in freshwater).
Chemical media is optional. Most healthy tanks never need it. It earns its place after medicating (to remove the drug), when tannins from driftwood tint the water and you want it clear, or when a smell needs tracking down.
Types of aquarium filters compared
There is no single best filter. Each type trades media capacity, cost, effort and behaviour when the power goes off. Prices below are indicative ranges for planning; check current listings before buying.
| Type | Best tank size | Media capacity | Upfront cost (₹ / $) | Maintenance effort | Noise | Shrimp/fry safe | Power-cut behaviour | Biggest downside |
|---|---|---|---|---|---|---|---|---|
| Sponge (air-driven) | 5-40 gal (20-150 L) | Low to medium | ₹150-800 / $3-10, plus air pump ₹300-1,000 / $5-15 | Low: squeeze out every 2-4 weeks | Air pump hum, bubbling | Yes, by design | Safe: sponge stays submerged in oxygenated water | Limited mechanical and chemical capacity; bubbles and takes up tank space |
| Hang-on-back (HOB) | 10-75 gal (40-280 L) | Low to medium | ₹800-4,500 / $10-55 | Low: rinse cartridge or media monthly | Low to moderate (waterfall splash) | Only with an intake sponge | Media in the box goes stagnant; some models need manual re-priming after restart | Small media volume; needs clearance behind the tank |
| Internal | 5-40 gal (20-150 L) | Low to medium | ₹500-3,000 / $7-35 | Low to moderate | Low, some vibration | With a sponge pre-filter | Submerged; water inside stagnates but stays wet | Uses tank space; motor sits in the tank |
| Canister | 30-200+ gal (110-750+ L) | High | ₹5,000-25,000+ / $60-300 | Moderate: deep clean every 2-3 months | Very low | With an intake sponge | Highest risk: sealed media sits without oxygen | Cost, hose disconnect for cleaning, leak risk |
| Sump | 75+ gal (280+ L), smaller with care | Very high | ₹8,000-50,000+ / $100-600+ (less if DIY) | Moderate to high | Variable: overflow gurgle, return pump | With overflow guard and pre-filter | Tank drains back to sump; overflow risk if undersized | Complexity, space, flood risk |
| Under-gravel (legacy) | 10-55 gal (40-200 L) | Gravel bed as media | ₹300-1,200 / $5-15 | Hard to clean without a teardown | Air pump hum | Yes | Air-driven; gravel bed stays wet | Clogs with debris, poor with rooted plants; largely replaced |
Under-gravel filters are included as a legacy note. You will still find them in old tanks and second-hand kits, but a sponge or HOB filter is a better choice for a new setup.
Shrimp and fry safety
Shrimplets and fry are drawn to intake slots and can be sucked against the grille or into the impeller. Any filter that pulls water through an open tube (HOB, internal, canister, sump) should have a foam sleeve or pre-filter sponge over the intake. Air-driven sponge filters are the safest option and are the standard choice for breeding tanks, shrimp tanks and quarantine.
Filter running cost and electricity
Running cost is worth checking because a filter runs 24 hours a day. Monthly energy is watts x 0.72, in kWh. Multiply by your tariff (roughly ₹7-9 per unit for many Indian households; check your own bill).
- Air pump for a sponge filter (3-6 W): about 2-4 kWh a month, roughly ₹15-35.
- HOB or internal filter (5-15 W): about 4-11 kWh, roughly ₹30-100.
- Canister filter (10-25 W for small to mid sizes): about 7-18 kWh, roughly ₹50-160.
- Sump return pump (20-60 W): about 14-43 kWh, roughly ₹100-390.
Check the wattage on the label of your specific model; these ranges are only a guide.
What happens to each filter in a power cut?
Power cuts are common in many parts of India and are rarely covered in filter guides. Water in a tank without flow loses oxygen slowly, but water sealed inside a filter loses it much faster.
- Sponge filter: the safest. The sponge stays submerged in tank water. A battery-backed or rechargeable air pump keeps flow going during longer cuts.
- HOB and internal: short cuts are fine. The media stays wet but stagnant. Check that the HOB restarts and re-primes when power returns.
- Canister: the most exposed. Sealed media with no flow can turn oxygen-starved within a few hours, and a rotten-egg smell on restart signals that die-off has occurred. If a cut has run long, flush the canister output to a bucket for a minute before returning water to the tank.
- Sump: when the return pump stops, water drains from the display tank back to the sump until the siphon breaks. The sump must have enough empty volume to take that drain-down without overflowing. Test it deliberately, with the pump off, before you rely on it.
How to choose the best aquarium filter for your tank size
Start with tank volume, then stocking, then plants and fish temperament.
- Up to 10 gallons: a sponge filter or a small internal filter. Sponge is the best pick for shrimp, bettas and fry.
- 10-20 gallons: a sponge filter, a HOB or a small internal filter. For the best aquarium filter for a 20 gallon, a HOB rated for 20-40 gallons or a sponge filter (or both) suits most community tanks.
- 20-55 gallons: HOB for straightforward setups, canister for planted tanks and messy fish.
- 55 gallons and up, or heavy bioload: canister or sump. Goldfish and large cichlids produce far more waste than their size suggests, so size up.
Do I need a canister filter? Not for most tanks under about 40 gallons. A canister is worth considering for a large tank, a planted tank that needs more media, heavy stocking, or if you want near-silent operation. Otherwise a HOB or sponge filter is simpler and cheaper.
Filter turnover rate and GPH: how much flow do you need?
Turnover is how many times per hour the filter moves a full tank volume. GPH (gallons per hour) is the rating printed on the box; many Indian sellers list litres per hour (L/h) instead. 1 GPH is about 3.8 L/h.
There is no single correct turnover. Commonly used ranges are 4-6x for a typical freshwater community tank, up to about 8-10x for heavy stocking, and lower for fish that dislike current. Media volume matters as much as flow: a small filter pushing 10x turnover through a thimble of media is worse than a larger filter at 5x with room for biological media.
Worked example
You have a 40-gallon (about 150 L) planted community tank.
- Tank volume: 40 gallons.
- Target turnover: 4-6x per hour for a planted community tank.
- Target real flow: 40 x 4 = 160 GPH, up to 40 x 6 = 240 GPH (about 600-900 L/h).
- Allow for rated vs real flow: if real flow is about 60% of the rated figure (see below), divide by 0.6. 160 / 0.6 is about 270 GPH and 240 / 0.6 is 400 GPH.
- Shopping range: a filter rated at roughly 270-400 GPH (about 1,000-1,500 L/h) with enough media space.
Use the Filter GPH Calculator to run the same steps for your own tank.
Rated vs real flow
The GPH on the box is measured with an empty, unrestricted outlet. In your tank the filter also has to lift water, push through tubing, elbows, lily pipes and spray bars, and flow through media that gets dirtier every week. Real flow is often well below the rating, commonly somewhere around 50-70% for a clean canister with a typical hose run, and lower again as media clogs. Treat the box figure as a ceiling and size with margin.
Flow for bettas, planted tanks, shrimp and goldfish
- Bettas: gentle. Long fins and light bodies struggle in strong current. Roughly 2-4x turnover, with the outflow baffled or spread with a spray bar, or a sponge filter turned down.
- Planted tanks: about 4-8x. Enough to move nutrients and CO2 around and stop dead spots, with the spray bar aimed to avoid uprooting plants.
- Shrimp: about 3-5x with a sponge-covered intake. An air-driven sponge filter is the usual choice.
- Goldfish: heavy bioload, so roughly 6-10x on the filter's rating with lots of media. Fancy goldfish are slow swimmers, so diffuse the outflow.
Filter media order: sponge, ceramic rings, carbon
Water should pass through the media in order from coarse to fine to biological to chemical:
Dirty water in
-> 1. Coarse sponge (largest debris)
-> 2. Fine sponge, floss or pad (fine particles)
-> 3. Ceramic rings / sintered glass (biological)
-> 4. Activated carbon or Purigen (optional, chemical)
Clean water out
Mechanical media goes first so debris does not clog the biological media. Carbon, if used, goes last. If your filter has limited space, prioritise: (1) sponge, (2) biological media, (3) everything else. Chemical media is the first to drop.
Sponge filter vs canister filter
The two are a good illustration of the trade-offs above.
A sponge filter is cheap, silent in operation apart from the air pump, and safe for fry and shrimp. It gives excellent biological filtration and survives a power cut. It offers little mechanical capacity for large tanks and no meaningful chemical filtration.
A canister filter holds a lot of media, gives flexible stages, is nearly silent and keeps equipment out of sight. It costs more, takes longer to clean, and is the most vulnerable to a long power cut.
Many keepers run both: a canister for capacity and a sponge filter as a backup that keeps a bacterial colony alive if the canister fails.
How to clean a filter without killing bacteria
Chlorine and chloramine in tap water kill the bacteria you have spent weeks growing. Always rinse media in a bucket of old tank water from a water change, or in dechlorinated water. Never scrub sponges and ceramic media until they are white; a light squeeze or swish is enough.
Do not clean everything at once. Stagger jobs so that some biological media is always left undisturbed. Never replace all biological media in one go.
Maintenance calendar
| Interval | Task |
|---|---|
| Weekly | Check flow, listen for noise, look for debris around the intake |
| Every 2-4 weeks | Rinse mechanical sponge or floss in old tank water; replace carbon if you are using it |
| Monthly | Check the impeller and intake tubes; clean sponge filters |
| Every 2-3 months | Deep clean canister, staggering media over separate weeks |
| Every 6-12 months | Replace worn sponge that no longer holds its shape; rinse clogged ceramic media |
When is carbon actually useful? After medicating, for tannin or odour control, or for short-term polishing. It is not required for a healthy tank to work.
Filter troubleshooting: noise, weak flow, cloudy water after cleaning
Noise. A rattling or grinding filter usually has an air pocket, a worn or dirty impeller, or a loose impeller cover. Low water level in the tank makes HOB filters splash and gurgle. A filter vibrating against glass can be cushioned with a small foam pad.
Weak flow. The usual causes are clogged sponge or floss, a dirty impeller, kinked hoses, a blocked intake, or air trapped in the system. Clean the mechanical stage first, then check the impeller and hoses. If flow is still weak with everything clean, the motor may be failing.
Cloudy water after cleaning. Cloudiness after cleaning often comes from disturbed debris or a bacterial bloom after you removed too much biological media. Wait a few days, do not clean the filter again, and test ammonia and nitrite. If either reads above zero, do a partial water change. Read the cloudy water guide for more.