What size filter do I need for my tank?

Reviewers repeatedly caution that manufacturer gallon ratings are optimistic. High-trust r/Aquariums threads flag that filters like the Fluval 407 rated for 50-100 gallons are better treated as adequate for the lower end of that range, especially with messy fish like goldfish or cichlids. A common rule cited by hobbyists is to choose a filter rated above your actual tank volume.