Do I really need a filter?

For essentially all fish, yes. The filter houses the bacteria that turn toxic fish waste into safer nitrate and keeps the water moving and oxygenated. A tank without a filter relies entirely on manual water changes to stay safe — hard to sustain, and unforgiving of any slip.

What size filter do I need?

Size by turnover: aim for a filter rated to move roughly 4–6 times your tank's volume per hour. For a 40-gallon (150 L) tank, that's about 160–240 GPH. Gentle-flow fish like bettas prefer the lower end (or a sponge filter); most community tanks are happy in the middle.

Which filter type is best for a beginner?

A sponge filter (cheap, gentle, superb biological filtration, safe for shrimp and fry) or a hang-on-back / HOB filter (easy, good surface agitation, simple media) are both excellent starter choices. Canister filters are powerful and hidden but are more filter than most small beginner tanks need.

How do I clean a filter without crashing my tank?

Gently, and rarely. Only clean when the flow noticeably drops, rinse the media in a bucket of the old tank water you removed during a water change (never tap water — chlorine kills the bacteria), and never replace all the media at once. Swap parts gradually so the colony survives.

Can I turn the filter off at night?

No. The bacteria in your filter need constant flow and oxygen; switching it off for hours starves them and lets waste build up. Leave the filter running 24/7 — it's also part of how the tank stays oxygenated overnight.

Aquarium filters explained: what they actually do, which type to pick, and the mistake that kills your bacteria

The 30-second version. A filter’s real job isn’t to polish the water clear — it’s to be the home for the bacteria that detoxify fish waste (the nitrogen cycle). That’s why you never turn it off, never rinse it in tap water, and never replace all the media at once — you’d be evicting your cleanup crew. Pick a sponge or hang-on-back filter for a first tank, size it to move ~4–6× the tank volume per hour, and keep it running always.

Most beginners think a filter is like a pool skimmer — a gadget that keeps the water looking clean. It does do that. But its clear-water job is the least important thing it does. Understand what a filter is really for and half of fishkeeping suddenly makes sense.

The three jobs of a filter

Filtration comes in three kinds, and a good filter does all three:

Biological (the important one). The filter media is a giant surface for the beneficial bacteria that convert toxic ammonia → nitrite → nitrate. This is your life-support system — it’s where the nitrogen cycle lives. Without it, waste poisons the fish.
Mechanical. Sponges, pads and floss physically trap floating debris — uneaten food, detritus, plant bits — which is what actually makes the water look clear.
Chemical. Media like activated carbon adsorb dissolved impurities, odours and tints. Useful, optional, and needs replacing periodically.

The reason this matters: the water can look crystal clear while the biology has crashed and ammonia is climbing. “Looks clean” is not “is safe.” The filter’s clear-water job and its keep-fish-alive job are different things.

The main filter types

Type	How it works	Best for
Sponge	Air pump pulls water through a porous sponge; bacteria colonise it, debris is trapped	Beginners, nano tanks, bettas, shrimp & fry — cheap, gentle, superb biological media
Hang-on-back (HOB)	Hangs on the rim; water is drawn up, through media, and spills back (good surface agitation)	Most beginner community tanks — affordable, easy to maintain, widely available
Canister	Sealed unit under the tank, large media volume, powerful flow via hoses	Bigger and heavily stocked tanks — high capacity, hidden, flexible media
Internal	Powerhead-driven unit inside the tank	Small-to-mid tanks, or as extra flow/filtration

For a first tank, a sponge filter or a HOB is almost always the right call. Bettas and shrimp especially like the gentle flow of a sponge — remember, a betta hates strong current.

Sizing: think turnover, not guesswork

Filters are rated in GPH/LPH (gallons or litres per hour). The simple rule is turnover: pick a filter that moves roughly 4–6 times your tank’s volume every hour.

A 40-gallon (150 L) tank → about 160–240 GPH.
Gentle-flow fish (bettas) → aim lower, or use a sponge filter.
Messy fish (goldfish, cichlids) → err higher; over-filtration is rarely a problem, under-filtration always is.

When in doubt, slightly over-filter — extra biological capacity is a safety margin.

The mistake that crashes tanks

Here’s the one that catches people out, and it flows straight from understanding what a filter is for: your bacteria live in the filter media, so how you treat that media is life or death for the tank.

Never turn the filter off overnight. The bacteria need constant, oxygenated flow. Hours off starves them (and drops the tank’s oxygen).
Never rinse media in tap water. Chlorine and chloramine kill the bacteria. Rinse it in a bucket of the old tank water you take out during a water change, and only when the flow noticeably drops.
Never replace all the media at once. That’s evicting the whole colony in one go and can trigger a mini-cycle. Swap parts gradually.
Don’t over-clean. A slightly “dirty” filter is a well-colonised filter. Clean for flow, not for looks.

Treat the filter as the living heart of the tank rather than a disposable gadget, and it will quietly keep your fish alive for years.

Where to go next

The biology your filter houses: the nitrogen cycle.
The other half of clean water: water changes.
Setting up from scratch: how to set up your first aquarium.

Sources & further reading

Buce Plant — Beginner’s Guide to Aquarium Filters and Types of Filtration — mechanical/biological/chemical filtration, the main filter types, and turnover sizing.
SeriouslyFish — species equipment figures (e.g. Poecilia reticulata) — per-species filter turnover guidance (~4–5× volume/hour).