Filtration is the most important equipment in any aquarium. Light controls plants and coral; the filter controls everything else - oxygenation, nitrification, particulate removal, water clarity. The right filter for a 10-gallon shrimp tank ($25 sponge filter) is wildly different from the right filter for a 120-gallon SPS reef ($800 sump + $400 skimmer).
Cost: $5-25. Sponge filters provide gentle biological filtration without sucking in shrimp larvae or fry. Drive with a small air pump ($15-30). Best for: 5-30 gallon tanks with delicate livestock.
Cost: $30-100. Aquaclear, Tidal, and Fluval HOB filters cover the standard freshwater community tank. Look for at least 4x tank turnover per hour. Easy to maintain, good biological + mechanical filtration. Best for: most freshwater tanks under 75 gallons.
Cost: $100-400. Fluval FX series, Eheim Classic + Pro series, Oase BioMaster. More media capacity than HOB, hidden under the tank for clean aesthetics. Best for: heavily planted tanks, oscars and other large freshwater fish, anything where extra biological capacity matters.
Cost: $150-1,000+ depending on size and complexity. A sump adds water volume, hides equipment (skimmer, heater, return pump), and provides a refugium for nutrient export. Drilled tanks with overflows route water down to the sump and back up via a return pump. Best for: any saltwater tank 30+ gallons, ideal at 50+ gallons.
Cost: $80-800. Removes dissolved organics that biological filtration alone can't handle at typical reef stocking densities. Hang-on-back skimmers (AquaMaxx HOB-1, Reef Octopus 110-INT) work for 30-65 gallon nanos. In-sump skimmers (Reef Octopus, Vertex, Royal Exclusiv) for sumped systems 50+ gallons.
Freshwater community: 4-6x tank volume per hour total flow. Planted tanks: 5-10x. Saltwater fish-only: 6-10x. Reef tank: 10-30x with multiple powerheads providing turbulent flow on top of the filter return.