Walk into any aquarium shop and the filters are sorted by a single number printed on the box: litres per hour, or a "suitable for tanks up to X litres" rating. Pick the one whose number is bigger than your tank, pay, go home.
That number is the least useful thing on the box. It is measured with an empty housing, no media, no hose and no lift — conditions that will never exist in your tank. Once you load it with sponge and ceramic and make it push water up thirty centimetres, real throughput can be well under half the sticker figure.
The question that actually decides which filter you should buy is not how many litres your tank holds. It is what is living in it, and how often you are realistically going to open the thing up.
What a filter is actually doing
Three jobs, and only one of them is the reason a filter exists.
Mechanical filtration traps floating particles. This is the visible job, the one that makes water look clear, and the one people over-value. Biological filtration is the real one: a filter is a box you have deliberately packed with surface area so that nitrifying bacteria have somewhere to live in enormous numbers, converting ammonia to nitrite and nitrite to nitrate. That colony is the single thing standing between your fish and ammonia poisoning. Chemical filtration — carbon, resins, phosphate removers — is optional and situational.
Everything below follows from that. A filter's value is mostly its media volume, not its pump. Filtration basics covers the mechanism in more detail; if that colony has never been established in the first place, no filter will save you — cycle the tank first.
Sponge filter
An air pump pushes bubbles up a lift tube, water gets dragged through a foam block, and bacteria colonise the foam. There is no impeller and nothing to break.
Buy it for: shrimp tanks, fry, quarantine and hospital tanks, bettas, and any nano tank. It cannot suck in anything small enough to be harmed, which is exactly why it is the default in a cherry shrimp tank and the only sensible choice for a betta, which hates being pushed around.
The local bonus: because it runs on air, a sponge filter is also aerating the tank. In a UAE summer, when warm water holds measurably less dissolved oxygen, that is not a side effect — it is half the reason to use one. The oxygen and air pump guide explains why this matters more here than in most places.
Against it: it does almost nothing mechanically, so fine debris stays in the water column. It is ugly. And it needs an air pump, which is a second device that buzzes.
Keep a spare sponge filter running in your main tank at all times. When you need a quarantine tank in a hurry, or a hospital tank because you are treating a disease, you lift out a fully cycled filter and the second tank is instantly ready. This one habit solves the most common emergency in the hobby.
Internal (submersible) filter
A sealed box with a pump on top, sitting inside the tank on suction cups. This is what comes in most all-in-one starter kits.
Buy it for: tanks up to roughly 60 litres with modest stocking, and situations where nothing can hang off the back — a tank pushed flush against a wall, or one inside a cabinet.
Against it: the media chamber is small, and in the cheapest models it is a single foam cartridge you are instructed to replace. Media volume is the whole game, and an internal filter is the format with the least of it. It also occupies space inside the tank, which in a small tank is space you wanted.
They are better than their reputation if you throw away the cartridge routine and pack the chamber with coarse sponge and ceramic media instead.
Hang-on-back
The default for most home aquariums between about 40 and 150 litres, and for good reason. It hangs on the rim, an intake tube pulls water up, the water passes through media in a chamber and pours back in over a lip.
For it: more media volume than an internal, nothing inside the tank but the intake, cheap, and serviceable in two minutes with wet hands and no bucket. That last point matters more than people expect — a filter you can open easily is a filter you will actually maintain.
The waterfall return also agitates the surface heavily, which is where gas exchange happens. In this climate that is a genuine advantage.
Against it, and this is the important part: most hang-on-back filters are sold with disposable cartridges. A cartridge is a slab of floss with a little carbon in it, and the instructions say to replace it monthly. Follow those instructions and you are throwing your bacterial colony in the bin every month and re-cycling the tank on a loop. This is behind a large share of the "my tank keeps crashing" and cloudy water complaints we see.
Do this instead: keep the plastic cartridge frame if you must, but pack the chamber with coarse sponge and ceramic rings, and never replace all of it at once. Rinse, do not renew.
The other local drawback is evaporation. A waterfall return into an open tank in a dry flat with the AC running will drop your water level fast, and as the water evaporates the minerals stay behind and concentrate. The lids article covers the trade-off, and the Dubai maintenance guide has the top-up rhythm.
Canister filter
A sealed bucket that lives in the cabinet below, connected by two hoses. Water is pulled down, forced through several litres of media, and returned.
Buy it for: anything from roughly 150 litres up, anything heavily stocked, and planted tanks where you want no equipment visible in the display. An African cichlid tank or a large arowana setup is canister territory, not because of flow but because those tanks produce a waste load that needs serious media volume.
For it: media volume in a completely different class, near-silent, nothing in or on the tank except two pipes, and the return can be aimed precisely — which matters in a planted tank, where you want gentle circulation everywhere and a still surface.
Against it: expensive. Servicing means shutting valves, disconnecting hoses and carrying a heavy wet bucket, so it becomes a job you postpone. And every canister is a set of rubber seals under pressure sitting inside your cabinet — replace the O-rings when they stop being supple, and grease them at every service.
If your tank is big enough that you are weighing a canister against a sump, that comparison has its own article.
Undergravel filters: skip them
A plate under the gravel with a lift tube pulling water down through the substrate. It works, in the narrow sense that it moves water and grows bacteria, but it turns your entire substrate bed into a debris trap you cannot clean, and it is incompatible with sand.
Sand is what you need if you keep corydoras or other bottom dwellers, and the substrate guide explains why fine substrate is the better default anyway. The two requirements do not coexist.
Sizing it without falling for the flow number
The working rule is four to six times tank volume per hour in circulated turnover — so a 100 litre tank wants a filter genuinely moving 400 to 600 litres per hour. Because the box figure is optimistic, choose a filter rated well above that.
Then stop thinking about flow. Between two filters with similar throughput, take the one with the bigger media chamber every single time. And remember that turnover assumes the fish load the tank was designed for — the stocking guide and the tank size guide are the upstream decisions that determine what filter you need. Work out your actual litres before you shop; a tank's advertised size and its real water volume are not the same number.
The UAE variables
Heat. Warm water holds less oxygen, and a filter's surface agitation is a meaningful part of your gas exchange. In July, point the return so it breaks the surface. The summer cooling guide covers the rest.
Hard water. Our tap water leaves calcium scale on impellers and inside intake tubes, and a scaled impeller loses flow quietly over months. Pull the impeller at every second service, and soak it in diluted white vinegar when it looks crusty. The tap water guide explains what is actually in it.
Power cuts. A filter that has been off for hours in a hot flat is not just idle — its bacteria have been sitting in stagnant, deoxygenated water. The emergency plan article covers what to do, and it is also the honest answer to whether you can switch the filter off at night. You cannot.
Maintenance, which matters more than the model
Rinse media in a bucket of water you just siphoned out of the tank during a water change. Never under the tap — chlorine and chloramine in UAE municipal water will kill the colony you are trying to protect, which is the same reason you dechlorinate every drop of replacement water.
Replace media in stages, never all at once. Squeeze coarse sponge, do not scrub it. And if you are hoping a filter upgrade will fix an algae problem, it will not — algae is a light and nutrient balance issue, and a UV steriliser does not fix it either.
Quick answers
Nano tank, shrimp or a betta? Sponge filter, driven by a small air pump.
First 60 litre community tank? Hang-on-back, with the cartridge routine ignored and the chamber packed with sponge and ceramic.
100 litres or more, planted or heavily stocked? Canister.
Can I run two filters? Yes, and it is a good idea on any tank you care about. Redundancy if one fails, and a second cycled filter on hand for emergencies.
Do I need carbon? Not routinely. Use it to strip medication after treatment, then take it out.
My filter is running but ammonia is climbing. The filter is not the problem — the colony is. Check what your test numbers are telling you, and if fish are already suffering, start with the diagnostic guide.
The bottom line
Match the format to the livestock and to how much maintenance you will honestly do, then buy the biggest media volume in that format you can afford. Sponge for small and delicate, hang-on-back for the standard community tank, canister when the tank gets big.
What you must not do is buy on the litres-per-hour figure, or follow the cartridge-replacement instructions on the box. Those two mistakes cause more dead fish between them than any wrong choice of filter type. If you are still assembling the shopping list, the beginner setup checklist puts the filter in context — and the other piece of equipment that is bought on the wrong numbers entirely is the light, which has its own guide.