The Three Types of Aquarium Filtration
All aquarium filtration works through three mechanisms: mechanical filtration (physically trapping particles like fish waste and uneaten food), biological filtration (beneficial bacteria converting toxic ammonia → nitrite → nitrate through the nitrogen cycle), and chemical filtration (activated carbon and specialized resins removing dissolved chemicals, medications, and odors).
Every aquarium filter performs all three functions to some degree, but different filter types excel at different aspects. Understanding the three types of filtration helps you choose the right filter and maintain it correctly — cleaning a biological filter media too aggressively destroys the bacteria colony you depend on for safe water.
Biological filtration is the most important of the three. Without it, ammonia accumulates to lethal levels within days. Mechanical and chemical filtration are secondary — beneficial but not immediately life-sustaining. When evaluating a filter, ask first: how much biological media capacity does it have?
Sponge Filters — Best for Bettas, Fry, Quarantine
A sponge filter is a foam block connected to an airline and air pump. Water is drawn through the sponge by the air lift, trapping particles in the foam while the enormous surface area of the sponge pores colonizes beneficial bacteria. They are inexpensive ($2-8 USD), gentle in flow (perfect for betta fins and fish fry), and extremely reliable with no mechanical parts to fail.
Sponge filters are the recommended choice for betta tanks, breeding tanks, fry tanks, quarantine tanks, and any tank under 60 liters. Their main limitation is that they do not filter polishing (water clarity) as well as canister filters, and they do require an external air pump.
Clean sponge filters by squeezing them gently in a bucket of used tank water (never tap water — chlorine kills the bacteria colony) every 2-4 weeks. Never clean more than 50% of your sponge filter media in a single maintenance session.
Hang-On-Back (HOB) Filters — Best for Community Tanks
HOB filters hang on the back of the tank, drawing water up a siphon tube and through internal media (usually a combination of mechanical floss, activated carbon cartridge, and optional bio rings or balls). They are convenient, easy to maintain, visible for monitoring, and aerate the water as they return it over the outflow weir.
HOB filters are the most popular choice for medium community tanks (40-200 liters). Brands like Aquaclear, Fluval, Sunsun, and EHEIM Aquacompact offer excellent models at a range of price points. The Aquaclear series is particularly popular among hobbyists because the filter media is customizable and the flow rate is adjustable.
Do not replace the entire filter cartridge at once during maintenance — this destroys your biological colony. Replace only the mechanical media (filter floss) monthly; leave the biological media (ceramic rings, bio sponge) undisturbed for 6-12 months, only rinsing gently in used tank water when flow becomes restricted.
Canister Filters — Best for Large Tanks and Discus
Canister filters are external units that sit below the tank, draw water through intake and output tubes, pass it through multiple media chambers inside the sealed canister, and return it to the tank. They offer the largest media volume of any filter type, the most customizable media configuration, and the quietest operation.
Canisters are the preferred choice for tanks above 100 liters, planted tanks where surface agitation must be minimized (to retain CO2), discus tanks requiring pristine water, and any setup demanding maximum biological filtration capacity. EHEIM, Fluval FX, and Sunsun produce widely available canisters in Cambodia.
Canister filter maintenance is less frequent (every 6-8 weeks) but more involved than HOB maintenance. Always turn off the filter before opening, keep a towel ready for spills, and service the mechanical media first (rinse in used tank water) before moving to the biological media chambers (ceramic rings, bio balls — rinse very gently, never clean all media at once).
Internal Filters and Undergravel Filters
Internal power filters are compact, submersed units common in starter tank kits. They are adequate for small tanks (under 40 liters) but limited in biological capacity and can be difficult to customize. They are better than nothing but should be upgraded as the tank grows or the fish load increases.
Undergravel filters (UGF) draw water down through the gravel substrate via lift tubes and air pumps. They were popular in the 1970s-1980s but are largely obsolete today. The substrate becomes the biological media, which sounds efficient but creates massive maintenance problems — detritus accumulates under the gravel plate and must be deep-cleaned periodically. Not recommended for new setups.
- ✦Always run a filter sized for at least 4× your tank volume in turnover per hour (e.g., 200-liter tank needs 800 L/h filter)
- ✦Never clean all filter media at once — stagger maintenance between chambers
- ✦Rinse all media in old tank water, never tap water
- ✦Run a sponge filter alongside a canister or HOB for redundant biological filtration
- ✦If the filter stops unexpectedly, restart it as soon as possible — the bacteria colony begins dying within hours without water flow