One of the biggest myths in the aquarium hobby is that a canister which has an actual flow rate of 600 GPH (gallons per hour) can filter roughly twice the amount of fish that a canister that has an actual flow rate of 300 GPH. In actuality the 600 GPH canister might be 10% better in filtration than the 300 GPH canister, assuming both filters have the same volume and media.
The concept that flow is important was tested. It showed doubling the flow rate added some 9% to the filter capacity. All the well meaning but ill-informed commentators on social media say it should double the capacity. This test can be found at this link:
8.3.1. Canisters in Depth (links to aquariumscience.org)
Experimentation on recirculating aquaculture systems (RAS) for farming tilapia have shown that flow rate does influence the ammonia oxidizing ability of a filter. But the relationship is not linear. On a basis of ammonia oxidizing capability per square foot of surface, doubling the flow rate through a canister filter will add 5% to 15% to the filtration capacity of the filter. The Europeans are quite successful with using much lower flow rates through their canisters than Americans use.
A typical canister filter will require very roughly 5 square feet of surface area to oxidize the ammonia of one pound of fish. A fluidized bed filter will only require roughly 2 square foot of surface area per pound of fish for ammonia oxidation. And an air operated sponge filter will require at least 10 square feet of surface area per pound of fish to oxidize ammonia. This is because of the flow rate through and in each filter. But it is a phenomenon called “turbulent flow”, which is more complex than just the flow rate through the filter.
For a more in depth analysis of this topic click the link below:
6.5.1. Flow Function (links to aquariumscience.org)
Another common myth is that the substrate in an aquarium does a lot of biofiltration. Beneficial bacteria require two things to live and oxidize ammonia well. The first is a surface to grow on and the second is decent water flow over that surface. While the substrate provides a lot of surface area it has little flow. So it doesn’t do a lot of filtration.
The idea that substrate is important as a place for beneficial bacteria to grow was tested. The tests and the scientific literature say the substrate in even a poorly filtered aquarium only provides 10% to 20% of the filtration. The tests are at this site:
6.5.2. Substrate as a Filter (links to aquariumscience.org)