Filtration & Equipment 5 min read

Aquarium Filtration: Types, Sizing & Failure Fixes

By The Aquarium Calculators Editorial Team ·

How to choose, size, and troubleshoot aquarium filters for crystal-clear water and healthy fish.

Hang-on-back aquarium filter flowing water into a planted freshwater tank with clear glass

Quick Answer

Aquarium filtration is a mechanical, biological, and often chemical process that removes physical debris, converts toxic ammonia into less harmful nitrates, and polishes water clarity. The single most common failure point is an undersized filter providing fewer than four to six tank-volume turnovers per hour, which leaves fish waste and organic compounds to accumulate. Installing a filter rated for your exact tank volume—or one size larger—and seeding it with established biological media from a cycled tank will establish stable water quality within 48 to 72 hours. Treating only cloudy water with clarifiers without addressing flow rate or media maintenance will cause repeated ammonia spikes and fish stress.

The Problem

My aquarium water looks cloudy, smells musty, or my fish are gasping at the surface even though my filter is running.

When to Use This Tool

Use this diagnostic when water remains hazy after water changes, when filter output feels weak, or when you need to distinguish a true filtration problem from overstocking or bacterial bloom.

Who is this for?

  • Freshwater aquarium hobbyists selecting, installing, or troubleshooting filtration systems from nano tanks to 100-gallon setups.

Aquarium Filter Types Compared

Filter TypeBest ForFlow Rate (GPH)CostMaintenance
Canister Filter40+ gallon planted/community tanks200–400+$$$Monthly media rinse, quarterly deep clean
Hang-on-Back (HOB)10–55 gallon community tanks100–300$$Bi-weekly cartridge check, monthly media swap
Sponge FilterBreeding, shrimp, quarantine tanks20–80 (air-driven)$Bi-weekly squeeze in tank water
Internal Power FilterNano tanks up to 20 gallons40–100$Weekly sponge rinse, monthly impeller check
Sump/Refugium75+ gallon or marine setupsCustom (500+)$$$$Weekly sock change, monthly pump maintenance
Troubleshooting Guide
1 Filter is running but water remains cloudy and smells musty.

Check for filter bypass caused by clogged mechanical media; rinse or replace filter floss/sponge and ensure water flows through all media baskets rather than around them.

2 Water cleared after a filter upgrade but turned cloudy again within a week.

The new filter lacked established biological media; seed it with ceramic rings or bio balls from a cycled tank and test ammonia daily until readings hold at 0 ppm.

3 Fish gasp at the surface despite a strong filter outflow.

The filter may be creating surface turbulence without adequate oxygen exchange, or the tank is overstocked beyond the filter's biological capacity; increase surface agitation or reduce bioload.

4 Filter motor is loud or rattling after cleaning.

Reassemble the impeller and housing correctly, ensure the impeller shaft is not bent, and add a thin rubber pad beneath the unit to dampen vibration.

Frequently Asked Questions

What causes an aquarium filter to stop working even though the motor is running?
Filter bypass is the most common cause. When mechanical media becomes clogged with debris, water channels around the filter pads or baskets instead of flowing through them. The motor runs, but water is not being cleaned. Rinse or replace the mechanical media and check that all intake slots are unobstructed.
How often should I clean my aquarium filter?
Mechanical media should be rinsed or replaced every two to four weeks, or sooner if flow drops. Biological media only needs gentle rinsing in siphoned tank water when mechanical media is serviced. Chemical media such as activated carbon must be replaced every three to six weeks before it releases adsorbed compounds back into the water.
Can you over-filter an aquarium?
You cannot have too much biological or mechanical filtration, but excessive flow velocity can stress slow-moving fish like bettas or gouramis. In planted tanks, high flow can also strip CO2 from the water column. Aim for four to six turnovers per hour for community tanks, and up to ten for heavy bioload species like goldfish.
Do fish tanks need a filter if I do frequent water changes?
Even with daily water changes, an aquarium filter provides continuous oxygenation, surface agitation, and stable biological processing that buckets cannot replicate. Without a filter, ammonia spikes between water changes can damage fish gills and suppress immune function.
Why is my new filter making my water cloudy?
A new filter often lacks sufficient nitrifying bacteria to process the existing bioload. This causes a bacterial bloom in the water column while the filter media colonizes. Seed the new filter with established biological media from a cycled tank and test ammonia and nitrite every 24 hours until both read zero.
Can I rinse my filter media under tap water?
No. Tap water contains chlorine and chloramine that kill nitrifying bacteria on biological media. Always rinse mechanical and biological media in siphoned tank water or dechlorinated water to preserve the colony. This is the shortcut everyone takes, and it is why tanks crash after filter cleaning.
Is a bigger filter always better?
A larger filter provides more biological capacity and mechanical surface area, but it must be matched to the tank's actual flow needs. A filter rated for 100 gallons on a 10-gallon tank may create excessive current that stresses fish and uproots plants. Choose a filter rated for your tank volume or one size up, not three sizes up.
How do I know if my filter is the right size for my tank?
Calculate the turnover rate by dividing the filter's GPH rating by your tank volume in gallons. Freshwater community tanks need four to six turnovers per hour. If your 40-gallon tank runs a filter rated at 100 GPH, you are only achieving 2.5 turnovers and need additional filtration or a stronger unit.
Glossary of Terms
Mechanical filtration
The physical removal of particulate waste such as uneaten food, feces, and plant debris through porous media like sponge, floss, or mesh screens.
Biological filtration
The cultivation of nitrifying bacteria on high-surface-area substrates to convert toxic ammonia and nitrite into less harmful nitrate.
Turnover rate
The number of times the total aquarium water volume passes through the filter in one hour, typically expressed in gallons per hour (GPH).
Filter media
The materials placed inside a filter to perform mechanical, biological, or chemical filtration, including floss, ceramic rings, activated carbon, and bio balls.
Chemical filtration
The use of adsorptive media such as activated carbon or resins to remove dissolved organic compounds, medications, or specific ions from water.
Scientific References
  1. Aquaculture Engineering: Filtration and Water Treatment
  2. Standard Methods for the Examination of Water and Wastewater
  3. The Ecology of the Planted Aquarium
  4. Aquarium Filtration: Biological, Chemical, and Mechanical Processes