Filter size is not only about the number on the tank box. Use your working water volume, the fish you keep, the flow they tolerate, and the manufacturer rating for the exact filter.
Start with the actual aquarium
Decorations and substrate reduce water volume, while active fish, heavier stocking, and waste-producing species can change filtration needs. A filter should move and clean water without creating a current that stresses the fish.
Read the manufacturer rating
Use the manufacturer tank-size range as the starting point, then consider the room for media and whether the flow can be adjusted. Aqueon recommends choosing a filter rated at least one size larger than the aquarium for added capacity, but product instructions still take priority.
Check the result after setup
Watch flow, temperature distribution, debris collection, and water-test results. A filter cannot correct overstocking or replace regular maintenance. See How Often to Change Fish Tank Water and Aquarium Filter Media Order for the next steps.
