Filter choice depends on tank size + bioload + livestock type. Hang-on-back (HOB): AquaClear, Seachem Tidal, Marineland Penguin. Best for 10-40 gallon tanks. Pros: cheap ($25-80), easy maintenance, customizable media. Cons: visible from outside, water-evaporation noise, limited capacity. Canister: Fluval 07 series, Eheim Classic + Pro4, Oase BioMaster. Best for 40-150 gallon tanks. Pros: 4-stage media customization, silent, hidden, large capacity. Cons: $130-400, complex maintenance, can leak if seals fail. Sponge filter: Hikari Bacto-Surge, Hydro Sponge. Best for shrimp tanks, fry tanks, hospital tanks. Pros: $5-20, gentle flow won't suck up shrimp/fry, biological powerhouse, near-zero maintenance. Cons: ugly, no mechanical filtration. Internal: Tetra Whisper, Fluval U-series. Best for budget setups + breeder tanks. Pros: cheap, all-in-one. Cons: takes up tank space, low capacity. Reef sump: Trigger Systems, EShopps. Best for reef tanks 75g+. Pros: hide skimmer + heater + ATO, dramatically increased water volume, refugium space. Cons: $200-1,000, requires drilled tank or overflow box, plumbing complexity. Capacity rule: filter rated GPH should be 4-6x tank volume for FW, 8-15x for SW.
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