Aquarium filters: media, flow, and the maintenance that crashes a tank

Quick answer

A filter is not a strainer. Its most important job is to hold the bacterial colony that converts ammonia into nitrite and nitrite into nitrate, and that colony lives on the surfaces of the media inside it.

That single fact decides almost every choice you make. It is why rinsing a sponge under the tap can crash a tank that has been stable for years, why the cartridge-replacement schedule printed on the box is bad advice, and why a filter that has been switched off for a few hours must not simply be switched back on.

• Never rinse filter media in tap water. Chlorine and chloramine kill the bacteria you are trying to keep.
• Never replace all the media at once. Stagger it, and keep the old media in the filter alongside the new for weeks.
• Rinse mechanical media often and biological media rarely, and only in water taken out of the tank.
• A filter that has been off for hours holds oxygen-starved, decomposing water. Clean it out before restarting it.
• Match the flow to the fish. A betta or a fancy goldfish in a high-flow tank is exhausted, not exercised.

Decide in 60 seconds

What a filter actually does

Biological filtration is the real product.
Ammonia is produced continuously by fish and by decomposing food. It is toxic at very low concentrations. Two groups of bacteria growing as a film on surfaces inside the filter convert it to nitrite and then to nitrate, which is far less toxic and is exported by water changes.

The colony sizes itself to the amount of waste you produce. Adding more media does not create more bacteria; it gives the existing population room, resilience and somewhere to grow into if you add fish. Feeding heavily, adding fish quickly, or removing media all change the balance faster than the colony can adjust.

Mechanical filtration keeps waste in one place, which is only useful if you remove it.
Sponges and floss trap solids so the water is clear. That trapped material is still in the system. Left in place, it breaks down inside the filter and releases the ammonia you were trying to remove, and it slowly blocks flow. This is why mechanical media is rinsed often and biological media is barely touched.

Chemical filtration is optional and time-limited.
Activated carbon adsorbs dissolved organics and removes discolouration and some odours. It has a finite capacity, and when it is exhausted it simply stops adsorbing. It also removes most medications from the water, which is why it comes out before any treatment. Resins that target phosphate or ammonia have their own limits. None of these is a substitute for water changes.

Water movement is a service the filter provides incidentally.
Circulation prevents dead spots where debris settles and oxygen falls. Surface disturbance drives the gas exchange that puts oxygen in and takes carbon dioxide out. A filter that returns water below the surface with no ripple is doing far less for oxygen than one that breaks the surface.

Choosing the type

Sizing.
The usual convention is to choose a filter rated to turn over several times the tank volume every hour, commonly quoted as four to six times for a general community tank. Treat printed ratings as optimistic: they are measured with an empty filter and no media, no hoses and no head height. Choosing a filter rated for a larger tank than yours is normal practice.

Two smaller filters beat one large one.
Redundancy matters. If one fails, the tank is not left with nothing, and you always have a mature colony to seed a replacement or a hospital tank.

Matching flow to the fish

Turnover is not a virtue on its own. It has to suit the animals.

Low-flow species.
Bettas, fancy goldfish, gouramis, angelfish and most long-finned varieties evolved in still or slow water. In strong flow they hold position against the current continuously, struggle to reach food before it is swept away, and often sit behind decor at one end of the tank. Owners read the stillness as calm. It is exhaustion.

High-flow species.
Hillstream loaches, many danios, and riverine species need both current and the high oxygen that comes with it. In a still tank they become lethargic and prone to disease.

How to reduce flow without losing filtration.
Fit a spray bar and point it along the glass or at the surface. Deflect the output against the tank wall. Add a sponge prefilter to the intake. Reduce the valve on a canister rather than throttling the pump on the outlet side only.

How to increase oxygen without increasing current.
Aim the return so it dimples the surface, or add an air stone. Oxygen exchange happens at the surface; a strong jet aimed along the bottom moves a lot of water and exchanges very little gas.

The kit that matters is a siphon and a bucket. The water you take out during a water change is what you rinse filter media in

Intake safety

Small fish, fry and shrimp.
An uncovered intake is a hazard. Fry are drawn in, shrimp are drawn against the strainer and cannot free themselves, and a slow-moving fish that becomes unwell is often found stuck to the intake. A sponge prefilter over the strainer removes the risk entirely and adds biological surface area at the same time.

Long-finned fish.
Bettas, veiltail guppies and fancy goldfish catch fins in slots and tear them. Torn fins in clean water usually heal; torn fins in a tank with poor water quality become fin rot.

Snails and the impeller.
A snail drawn into a canister or power filter jams the impeller. The first sign is a change in the noise or a drop in output.

Sudden weakness.
Any fish found pressed against the intake needs a health assessment, not just rescuing. Healthy fish do not get stuck.

The maintenance that keeps the colony alive

Rinse media in tank water only.
Take a bucket of water out during your water change and squeeze the sponges in that. Tap water contains chlorine or chloramine specifically because it kills bacteria, and it does not distinguish between the bacteria in the pipes and the ones you have spent months growing.

Rinse, do not clean.
Squeeze until the worst of the debris comes out and the flow will be restored. A sponge that comes out looking new has lost most of its bacterial film. Brown and functional beats clean and sterile.

Stagger any media replacement.
When a sponge genuinely disintegrates, replace part of it, or add the new piece alongside the old for several weeks before removing the old one. The new medium needs time to be colonised from the old.

Separate your disturbances.
Do not clean the filter, gravel vacuum the substrate and do a large water change on the same day. Each one removes some bacteria and some buffering. Doing all three together is a common trigger for an ammonia reading in a mature tank. Alternate them across weeks.

Rinse mechanical media on a schedule, biological media on symptom.
Floss and fine pads clog quickly and should be rinsed or replaced often, because material trapped in them is decomposing. Ceramic media and mature sponges are touched only when flow has genuinely dropped.

Do not pack media bags tight.
Water takes the easiest route. A canister stuffed so tightly that flow channels around the media looks like it is running while filtering very little. Leave the media loose enough for water to pass through rather than past.

Service the impeller.
Pull the impeller out periodically. Grit, hair and mineral scale build up on the magnet and shaft, and ceramic shafts crack. A gradual drop in output with clean media is usually the impeller.

Look after the seals.
Canister O-rings need to be clean, correctly seated and lightly lubricated with the manufacturer's grease. A pinched or dried O-ring is the classic cause of a filter that empties itself onto the floor overnight.

:::danger
Never restart a filter that has been switched off for hours without cleaning it first.

Nitrifying bacteria need oxygen. Once flow stops, the oxygen inside the media is consumed within a short time, the colony begins to die, and the trapped organic waste starts decomposing anaerobically. The result is a volume of water containing ammonia and, in bad cases, hydrogen sulphide, which smells of rotten eggs.

Switching the filter back on pumps that directly into the tank. After any power cut, holiday, or filter that has been left off after cleaning, disconnect it, tip out the standing water, rinse the media in tank water, and then restart. If it smells foul, discard all the water inside it and expect the tank to need close monitoring for ammonia over the following days.
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Reading a filter that is going wrong

When you are replacing or upgrading a filter

Run both filters together.
Install the new one and leave the old one running for several weeks. Bacteria will colonise the new media from the water and from the old filter.

Better still, move media across.
Put a piece of mature sponge or a bag of mature ceramic media from the old filter into the new one. That is the single fastest way to establish a new filter, and the same trick sets up a hospital tank in an afternoon.

Test through the changeover.
Check ammonia and nitrite through the transition, not just at the end. A problem caught at a trace reading is a water change; a problem caught by fish behaviour is already established.

Do not upgrade and restock in the same week.
The new capacity does not exist until the bacteria do.

What changes with the setup

Heavily stocked or messy fish.
Goldfish, koi and large cichlids produce far more waste per fish. They need more mechanical capacity, more frequent rinsing and more water change volume, not just a bigger box.

Planted tanks.
Plants take up ammonia directly, which buffers the tank against small filter problems. That is a margin, not immunity, and a heavily planted tank still crashes if the media is destroyed.

Shrimp and fry tanks.
Sponge filters, or prefilters on everything. Shrimp are also far more sensitive to copper and to some medications, which changes what you can safely run in the chemical stage.

Marine and reef systems.
Biological processing is distributed across live rock and sand as well as the filter, and additional equipment such as a protein skimmer removes organics before they break down. Mechanical media left unrinsed in a reef system becomes a nitrate source quickly.

Ponds.
Volume and temperature swing far more, the bacterial colony slows in cold water, and winter management is a separate discipline. Do not clean a pond filter thoroughly at the start of the season when the colony is at its weakest.

Small tanks.
The smaller the volume, the faster anything goes wrong. A filter problem in a large tank gives you days; in a very small tank it can give you hours.

What never to do

Do not rinse any filter media under a running tap.

Do not replace all the media at once, however dirty it looks.

Do not run a filter dry. Impellers depend on water for cooling and lubrication and burn out quickly without it.

Do not switch a long-stopped filter straight back on.

Do not leave activated carbon in during a course of medication. It will strip the treatment out and the fish will be dosed with nothing.

Do not add a new filter and a batch of new fish in the same week.

Do not use soap, detergent or any household cleaner on any part of a filter or on a bucket used for the tank. Residues are lethal in an aquarium and rinsing does not reliably remove them.

Do not judge a filter by how clear the water is. Crystal clear water can carry a dangerous ammonia level, because ammonia is invisible.

A stable tank is the visible result of a bacterial colony you have protected rather than scrubbed

When to get help

Act immediately, on the same day, if fish are gasping at the surface, hanging motionless near the outflow, or clamping their fins after any filter work or restart. Increase aeration, do a partial water change with conditioned water at the same temperature, and test ammonia and nitrite.

Get advice from an aquatic specialist or a vet who sees fish if ammonia or nitrite remain detectable for more than a few days despite water changes, if fish continue to die after a filter incident, or if a treated tank is not responding, which often means the medication is being adsorbed by carbon you forgot to remove.

Take your test results with you rather than a description. Ammonia, nitrite, nitrate, pH, temperature, tank volume, stocking list, when the filter was last serviced and what you did to it will identify the cause faster than anything else you can bring.