Aquarium Filtration: Flow Rate, Bioload, and Filter Type
Filtration is the life support system of an aquarium. Unlike most aquarium equipment, choosing the wrong filter — specifically, an undersized one — doesn't produce obvious symptoms until ammonia or nitrite reaches toxic levels. By then, the damage to fish health may already be significant.
The Turnover Rule
Filter flow rate is typically expressed as a "turnover" — the number of times the entire tank volume passes through the filter per hour. A 200L tank with a filter rated at 800 LPH achieves 4× turnover.
The standard recommendation of 4–6× turnover applies to lightly to moderately stocked community aquariums. This is not a biological constant — it's a practical guideline derived from the bioload of average community fish. Heavy bioload fish (goldfish, large plecos, heavily stocked cichlid tanks) require 8–10× turnover minimum.
Note that manufacturer-rated flow rates are tested without media resistance. Real-world flow through a fully loaded canister filter with multiple media stages is 20–40% lower than the rated figure. Always size up by at least one tier when buying a canister filter.
Biological vs. Mechanical Filtration
Filtration has three stages, and flow rate addresses mechanical filtration most directly:
- Mechanical: Physical removal of suspended particles via filter media. Flow rate determines how quickly particles are captured before settling. Regular cleaning (every 1–2 weeks for HOBs, 4–6 weeks for canisters) prevents nitrate accumulation in trapped waste.
- Biological: Colonization of filter media by beneficial bacteria (Nitrosomonas, Nitrospira). The biological capacity is determined by surface area of media, not flow rate — more porous media with higher surface area supports larger bacterial colonies.
- Chemical: Activated carbon, Purigen, zeolite — removes dissolved organics and specific chemicals. Optional for most setups; removed during medication treatment.
Filter Type Selection Guide
Canister filters are the best choice for most tanks above 100L. They have large media volumes for biological filtration, work silently, and can be customized with different media layers. The sealed design means water must pass through all media stages without bypassing.
HOB (hang-on-back) filters are excellent for tanks under 200L. Easy to access for cleaning, good flow, but limited media volume compared to canisters of similar footprint. The AquaClear series with sponge/BioMax/activated carbon is the standard recommendation for HOBs.
Sponge filters are ideal for fry tanks, hospital tanks, quarantine tanks, shrimp tanks, and betta tanks. Airstone-driven water flow is gentle enough for fry and shrimp. The sponge surface hosts biological filtration and provides microfilm (aufwuchs) that shrimp and fry graze on. They cannot handle heavy bioload tanks.
Sump filters are the professional choice for large tanks and saltwater reef systems. The separate sump chamber allows large media volumes, easy equipment placement (skimmers, reactors, dosing pumps), and maintenance without disturbing the display tank.
Goldfish: The Special Case
Goldfish filtration requirements deserve special mention because they are dramatically different from other freshwater fish. A common goldfish produces approximately 4× more ammonia per unit body weight than a similarly sized tropical fish. A 300L goldfish tank with three standard-sized goldfish requires filtration equivalent to a 1,200L community fish tank.
The only reliable approach for fancy goldfish indoors is a quality canister filter rated significantly above tank volume (Fluval 407 or 407 on a 200L tank, not a 307), plus weekly 30% water changes. No filter, regardless of rating, eliminates the need for water changes in a goldfish setup.