Start with the space you can service
A filter is a recurring job as well as a purchase. Before comparing gallon ratings, identify where the body will sit, how you will reach the media, and where water goes while you open it. Measure the tank rim, rear clearance or cabinet opening that the format needs.
A model can fall inside a manufacturer capacity range and still be a poor physical fit. Start with the filter catalog, then use the exact manual to confirm the installation.
An air-driven sponge filter
The sponge sits inside the tank and connects to an air source. That keeps the filter media in view and accessible, while using some display space. Budget for the air pump, airline and any required valve or fittings; the sponge by itself is not a complete powered system.
Check the sponge dimensions, lift-tube height and air-source instructions. Do not equate the sponge size with a universal stocking capacity. Small inhabitants and the desired current also affect the choice.
Choose the pump, airline and valve for a complete sponge-filter system.
A hang-on-back filter
A HOB places much of the filter outside the tank, usually across the rear rim. Measure the trim and space between the tank and wall. Leave room to lift the lid and media basket without moving the aquarium.
Compare the supplied media and replacement format. A reusable basket and a model that depends on a specific cartridge create different ownership tasks. Follow the current manual for priming and restart; a self-priming claim should belong to the exact model.
A canister filter
A canister moves the filter body into a cabinet or another approved location and connects it with hoses. That can clear the tank rim, but adds hose routing, valves, seals and a separate body to service. Check the manual’s height and routing conditions before deciding it fits.
Compare filter circulation with filter circulation, not another model’s unloaded pump output. Make room for opening and removing the media stack as well as storing the closed canister.
Choose a service routine you will keep
- Check the exact media supplied and what must be added.
- Confirm that replacement parts and the manual identify your model.
- Plan a place to catch drips and lay out wet components.
- Read the unplug, opening, cleaning and restart sequence before purchase.
Compare two filters with their maintenance and fit conditions visible side by side.