The filter is the most important piece of equipment in your aquarium. Most people buy one based on the number on the box — "suitable for tanks up to 50 gallons" — and never think about it again until something goes wrong.
That number on the box, by the way, is almost always optimistic. Filter manufacturers rate their products for lightly stocked tanks under ideal conditions. For a realistically stocked community tank, buy a filter rated for roughly twice your tank's volume.
But before we get into which filter to buy, it helps to understand what a filter is actually doing.
Mechanical filtration
Mechanical filtration is physical. Water passes through a material — usually foam, filter floss, or a sponge — that traps solid particles. Fish waste, uneaten food, plant debris. Without mechanical filtration this stuff just circulates in the water column and breaks down into ammonia.
The key to mechanical filtration is cleaning it regularly. A clogged mechanical filter stops working and becomes a site where trapped waste decomposes into ammonia — the exact thing you were trying to prevent. Rinse mechanical media in a bucket of old tank water every couple of weeks. Never rinse it under the tap; the chlorine will kill the beneficial bacteria living on it.
Biological filtration
This is the one that actually keeps your fish alive. Beneficial bacteria colonize your filter media and convert ammonia (toxic) to nitrite (also toxic) to nitrate (relatively harmless). This is the nitrogen cycle running in your filter.
Biological media needs surface area. Ceramic rings, bio-balls, sponge, or purpose-built media like Seachem Matrix all work by providing a large surface for bacteria to colonize. The more surface area, the more bacteria, the more ammonia the filter can process.
Never replace all your biological media at once. If you do, you essentially crash your nitrogen cycle and have to restart. Replace only a third at a time, spaced weeks apart, to preserve the bacterial colony.
Chemical filtration
Chemical filtration removes dissolved substances that mechanical and biological filtration can't catch. Activated carbon is the most common — it adsorbs tannins (the yellow tint from driftwood), medications, and other organic compounds. It also polishes water clarity noticeably.
Carbon needs to be replaced regularly — once it's saturated it stops working and can start leaching back what it absorbed. Every 4–6 weeks is a typical replacement schedule. Remove carbon before adding any medication to the tank; it will absorb the treatment before it can work.
Other chemical media include phosphate removers (useful for reef tanks and algae management) and zeolite (absorbs ammonia directly, mostly used in quarantine or emergency situations).
Types of filters
Hang-on-back (HOB) filters are the most common for freshwater tanks. They're easy to set up, easy to maintain, and do a good job with all three filtration types. The main limitation is they sit on the outside of the tank and the intake can be a hazard for small fish or fry.
Canister filters sit below the tank and push water through a sealed container of media. More filtration capacity, quieter, and completely hidden. More expensive and slightly more involved to set up and clean. Worth it for larger tanks or when aesthetics matter.
Sponge filters are air-powered, cheap, and extremely gentle on water flow. They're the best choice for fry tanks, shrimp tanks, or hospital tanks where you don't want strong currents. Biological filtration only — no chemical media. They look ugly but they work.
Internal filters sit inside the tank, usually in a corner. Common in starter kits. Adequate for small to medium tanks. The main downside is they take up space inside the tank.
Sumps are a separate tank underneath the display tank that houses all the equipment. Standard in reef tanks. Adds significant water volume to the system, hides equipment, and makes maintenance easier. More complex to set up.
The maintenance mistake most people make
Over-cleaning. A filter that's a bit dirty is usually fine. A filter that gets cleaned too thoroughly loses its bacterial colony and crashes the tank's nitrogen cycle. Unless your filter is so clogged that flow is visibly reduced, a partial rinse in old tank water is all it needs.
Set a reminder in Tanqly for filter maintenance. Every 2 weeks for the mechanical media, monthly for a partial biological media rinse. That's usually enough.