Quick Answer
Aquarium filtration works in three stages: mechanical (traps particles), biological (bacteria convert ammonia to nitrite to nitrate) and chemical (optional adsorption). Size flow as a range, commonly 4-6x tank volume per hour for a typical freshwater community tank, and remember real flow is often well below the box rating. Choose the type by tank size, stocking and power-cut risk.
Key Takeaways
- Biological filtration is the stage that keeps fish alive; mechanical and chemical stages support it.
- Size turnover as a range, not one number, and allow for real flow being lower than the manufacturer's GPH rating.
- Media volume matters as much as flow: choose a filter with room for enough biological media.
- Rinse media in old tank water, never tap water, and never clean or replace all biological media at once.
- Sponge filters are safest in a power cut; canisters carry the highest risk of oxygen-starved media.
Frequently Asked Questions
How often should I clean my aquarium filter?
Rinse mechanical sponges or floss every 2-4 weeks in old tank water. Deep clean a canister every 2-3 months, staggering media so some biological media is always left alone. Clean more often if flow drops or the tank is heavily stocked.
Do I need a canister filter?
Not for most tanks under about 40 gallons. A HOB or sponge filter is simpler and cheaper. A canister makes sense for large or planted tanks, heavy bioloads such as goldfish, or when you want near-silent operation and more media space.
What is the best aquarium filter for a 20 gallon tank?
A sponge filter or a HOB rated for roughly 20-40 gallons suits most 20-gallon community tanks. Choose the sponge filter for shrimp, fry or bettas, and add an intake sponge if you use a HOB with delicate fish.
Can I have too much filtration?
More filtration rarely hurts, except for fish that dislike current, such as bettas, discus and fancy goldfish. For those, diffuse the outflow with a spray bar or baffle, or use a sponge filter for gentle biological filtration.
How do I know when to replace my filter media?
Replace mechanical sponge when it no longer holds its shape or starts to fall apart, typically every 6-12 months. Replace carbon every 3-4 weeks if you use it. Ceramic biological media rarely needs replacing; rinse it in old tank water when it clogs.
Does a sponge filter provide enough biological filtration?
Yes. The porous sponge is an excellent home for nitrifying bacteria. It has limited mechanical capacity for large or messy tanks and no real chemical filtration, so it is best for small tanks, shrimp, fry and quarantine, or as a backup to a larger filter.
Why is my filter flow lower than the rated GPH?
The rated GPH is measured with no resistance. In a real tank, tubing, elbows, spray bars, lift height and dirty media all reduce flow, often to roughly half to two-thirds of the rating. Size your filter with margin and keep media clean.
What should I do with my filter during a power cut?
Sponge filters are safe. HOB and internal filters are fine for short cuts. A canister left sealed for many hours can go oxygen-starved, so on restart flush its output to a bucket before returning water to the tank. Sumps must have room to take the drain-down.