Filtration 101: what to know about filters and why it matters

An aquarium filter does three jobs: it removes debris, hosts beneficial bacteria that break down waste, and can remove dissolved chemicals. Without filtration, fish waste, uneaten food, and decaying matter would poison the water. Understanding how filters work helps you pick the right one and maintain it properly.

Why filtration matters

Fish produce ammonia through their gills and waste. Uneaten food and plant matter also break down into ammonia. Ammonia is toxic — even low levels stress or kill fish. The nitrogen cycle converts ammonia to nitrite (also toxic) and then to nitrate (less harmful in moderation). Beneficial bacteria live on filter media, substrate, and decor. The filter is usually the main home for these bacteria because it has high water flow and lots of surface area. A good filter also removes visible debris so the water stays clear and the bacteria aren’t smothered by sludge.

The three types of filtration

Mechanical

Mechanical filtration traps solid particles — fish waste, uneaten food, plant bits. Sponge, floss, and pads act like a sieve. Water passes through; debris stays behind. This media needs rinsing or replacing when it clogs. If it’s too dirty, flow drops and the filter becomes less effective.

Biological

Biological filtration is the bacteria that convert ammonia → nitrite → nitrate. These bacteria colonise porous media: ceramic rings, sponge, bio-balls, matrix. The goal is lots of surface area and steady water flow. Never sterilise biological media with tap water or bleach — you’ll kill the bacteria and crash your cycle. Rinse it in old tank water during water changes.

Chemical

Chemical media removes dissolved substances. Activated carbon absorbs tannins, medications, and some impurities. Purigen, zeolite, and phosphate removers target specific compounds. Chemical media is optional for most tanks; it’s useful when you need to remove meds, clear tinted water, or tackle a specific problem. Replace it according to the product — carbon loses effectiveness over time.

Flow rate and turnover

Filters are rated by flow (litres or gallons per hour). A common guideline is 4–6× tank volume per hour — e.g. a 100 L tank needs 400–600 L/h. That’s a starting point; heavily stocked tanks or messy fish may need more. Gentle species (e.g. bettas, long-finned fish) may need less flow or a baffled outlet. See filter types and comparing filters for how different designs deliver flow.

Media order

Water should hit mechanical media first (to catch debris before it clogs biological media), then biological, then chemical if used. That order keeps the bacteria fed and the chemical media from clogging too fast.

Maintenance

Rinse mechanical media when flow drops — in old tank water, not tap. Leave biological media alone except for gentle rinsing in tank water. Replace chemical media per the instructions. Never replace all media at once — you’ll remove your bacteria and cause an ammonia spike. Track your water parameters with a test kit or App-aquatic to spot problems early.

Quick takeaways

• Filters provide mechanical (debris), biological (bacteria), and optional chemical filtration.
• Biological filtration is critical — it runs the nitrogen cycle.
• Match flow rate to tank size and fish needs; 4–6× turnover is a common target.
• Maintain media without killing bacteria — rinse in tank water, don’t replace everything at once.