Hang-on-back filters are simple and good for small to mid tanks; canister filters hold more media and stay quiet under the stand for larger tanks; sponge filters are gentle and cheap for fry, shrimp and quarantine; sumps give the most capacity for big and reef systems. Many keepers size a freshwater filter to turn the tank volume over about four to six times per hour, adjusted for stocking and fish that prefer gentle flow.
The filter keeps water clear and hosts the bacteria that make a tank livable.
Types
| Type | Best for | Strengths | Watch for |
|---|---|---|---|
| Hang-on-back (HOB) | 5 to 75 gal | Simple, easy maintenance | Space behind the tank, surface agitation |
| Canister | 30 gal and up, planted | Large media capacity, quiet | More work to service |
| Sponge | Fry, shrimp, quarantine, small tanks | Gentle, cheap | Takes space inside tank |
| Internal | Small tanks | Compact | Limited media |
| Sump | Large and reef tanks | Most capacity, hides equipment | Plumbing, overflow, cost |
Sizing
Turnover = rated flow (gph) ÷ tank gallons. Rated flow drops once media and hoses are added. Fish from slow waters, such as bettas, need gentle flow.
| Tank | 4 to 6× turnover |
|---|---|
| 20 gal | 80 to 120 gph |
| 40 gal | 160 to 240 gph |
| 75 gal | 300 to 450 gph |
Media
| Media | Job |
|---|---|
| Mechanical (sponge, floss) | Traps debris |
| Biological (ceramic rings, sponge) | Houses bacteria |
| Chemical (carbon, resins) | Removes odors, color, some compounds |
Maintenance
Rinse media in old tank water, not tap water, which can kill bacteria. Replace media gradually.
New tank? Read the nitrogen cycle guide. Browse tanks and filtration.
Common questions
What size filter do I need for my aquarium?
Many keepers aim for 4 to 6 tank volumes per hour of rated flow for freshwater, more for heavily stocked tanks.
Canister or hang-on-back filter?
Hang-on-back filters are easier and cheaper; canisters hold more media and suit larger or planted tanks.
Are sponge filters good enough?
Yes, for small tanks, shrimp, fry and quarantine. They provide gentle flow and good biological filtration.
How often should I clean my aquarium filter?
Rinse mechanical media in tank water when flow drops. Avoid replacing all biological media at once.