Guide
How to keep your fishtank water crystal clear
Filtration, maintenance, and habits that keep aquarium water clear and healthy.
- Filter first: Run a filter rated for at least your tank volume, with mechanical media (sponge or floss) to trap suspended particles. Clean it when flow drops - but rinse media in old tank water, never tap water.
- Water changes beat everything: 20-30% weekly, with the substrate vacuumed and dechlorinated replacement water matched to tank temperature (within 1-2°C / 2-4°F).
- Overfeeding is the #1 cause of cloudy water: Feed only what disappears in two minutes, once or twice a day. Remove any leftovers after five minutes.
- Milky white cloudiness in a new tank is a bacterial bloom: It clears on its own in two to five days. Don't do an emergency water change - it just restarts the cycle.
- Green water means algae, not bacteria: The fix is blocking light, not scrubbing the glass. Reduce photoperiod to six to eight hours and check nutrients.
- Clear water doesn't mean clean water: Nitrates can sit at 80 ppm in a tank that looks spotless. Test regularly - don't go by appearance alone.
Why aquarium water goes cloudy in the first place
There are three distinct types of cloudiness, and they have different causes and different fixes. Treating the wrong one wastes time and can make things worse.
White or milky cloudiness is almost always a bacterial bloom - a sudden spike in free-floating heterotrophic bacteria. It's extremely common in tanks under six weeks old, but it can also happen after a big water change in a mature tank or after cleaning the filter too aggressively. The bacteria are harmless and the bloom usually collapses on its own once the tank stabilises.
Yellow or brown tinting is typically tannins leaching from driftwood or unsealed substrate. It won't harm your fish - many species like blackwater conditions - but if you want it gone, activated carbon in the filter will clear it within 24-48 hours.
Green water is a free-floating algae bloom (Chlorella and similar single-cell species). It's driven by excess light and dissolved nutrients - mainly nitrate and phosphate. You can't filter it out with standard mechanical media because the cells are too small. You need a UV steriliser, a blackout, or both.
Most "water clarifier" products you'll find at the fish shop are flocculants - they clump particles together so the filter can catch them. They fix the symptom, not the cause. If your filtration and feeding are dialled in, you shouldn't need them. If they're not, the clarifier will help for a day and the cloudiness will return.
Getting filtration right
The filter does three jobs: mechanical (trapping physical particles), biological (converting ammonia and nitrite via beneficial bacteria), and chemical (optional - activated carbon, etc.). For clear water, mechanical and biological filtration matter most.
Matching filter size to your tank
The standard rule is to turn over the tank volume four to five times per hour. A 100-litre tank needs a filter pushing at least 400 litres per hour. With heavily stocked tanks, aim for six to eight times. Under-filtering is the single most common reason tanks stay chronically cloudy - the filter simply can't process the waste load fast enough.
Multiply your tank volume by four for lightly stocked tanks; by six to eight for densely stocked community setups.
Mechanical media: what actually traps particles
Coarse sponge handles large debris. Fine filter floss or a polishing pad traps the finer particles that give water a milky haze. If your canister or HOB filter has multiple stages, put coarser media before finer - it extends the life of your polishing pads and reduces how often you need to clean them.
Most people get this wrong by cleaning the filter media under the tap. Tap water contains chlorine (or chloramine), which kills the beneficial bacteria living in your biological media. Always rinse media in a bucket of water you've just removed from the tank during a water change.
Replacing all your filter media at once - including biological sponges - crashes your nitrogen cycle. If you need to replace media, do it in stages: swap out one section, wait two to three weeks, then swap the next. See the nitrogen cycle explained if you're unsure how this works.
When to clean the filter
Clean when you notice a meaningful drop in flow rate, not on a fixed schedule. Some tanks need monthly cleans; heavily stocked tanks might need it every two weeks. Over-cleaning a filter that doesn't need it does more harm than good - you're disrupting an established bacterial colony for no benefit.
Water changes: the non-negotiable foundation
No filter removes dissolved waste - nitrates, phosphates, dissolved organics. Only water changes do. If you skip them, these compounds build up, feed algae, and eventually stress your fish even if the water looks clear.
For most community tanks, 20-30% weekly is the right rhythm. Heavily stocked tanks, tanks with large messy fish like goldfish or cichlids, or tanks with sensitive species benefit from two smaller changes per week rather than one large one. For a full breakdown of frequency by setup, see our guide on how often to do water changes.
Getting the technique right
Temperature matching matters more than most guides admit. Adding water that's more than 2°C (4°F) colder than the tank stresses fish and can trigger ich outbreaks in species like Ichthyophthirius multifiliis-susceptible tetras and barbs. Get a cheap thermometer and check before you pour.
Always dechlorinate. Chlorine dissipates if you leave water out for 24 hours, but chloramine (used by many water suppliers) does not. A standard dechlorinator like sodium thiosulfate handles chlorine; make sure yours also handles chloramine if your tap water uses it - check your water supplier's website.
Vacuum the substrate during water changes. A gravel vacuum (siphon tube with a wide head) pulls debris out of the gaps between substrate particles without removing the gravel itself. Over sand, hover about 2-3 cm above the surface rather than pushing in - you'll suck up the sand along with the waste otherwise.
If you have a deep substrate (over 5 cm), be careful about stirring it all at once. Anaerobic pockets in deep substrate can release hydrogen sulfide - that rotten-egg smell - which will cloud the water and can harm fish in high concentrations. Work the substrate in sections across multiple water changes rather than doing a full disturb in one go.
Feeding: the variable most people underestimate
Uneaten food is the fastest route to a cloudy tank. It starts decomposing within hours, feeding bacterial blooms and pushing ammonia up. Most community fish need less food than their owners think.
The two-minute rule is a reliable guideline: feed an amount your fish finish completely in under two minutes. For bottom-dwellers like Corydoras or plecos, add sinking wafers after lights-out and remove any uneaten pieces the following morning. For surface feeders, flake or micro pellets once or twice a day is usually enough.
If you're going on holiday, most healthy adult fish can go seven to ten days without food with no ill effects. An automatic feeder set to dispense small amounts is a better option than asking a neighbour who doesn't keep fish - well-meaning overfeeding during a week's holiday is a classic disaster scenario.
Feeding on a "daily schedule" without checking whether the previous meal was actually eaten. If there's still food sitting on the substrate when you feed again, you're overfeeding. Cut back immediately and do a small water change to remove the decomposing waste.
Fixing green water specifically
Green water doesn't respond to water changes or filter cleaning because the algae cells are far too small for standard mechanical media. You have three practical options:
Reduce light: Cut your photoperiod to six hours a day and block direct sunlight on the tank. Most algae blooms collapse within a week with light restriction alone. Once clear, settle on eight to ten hours of artificial light and stick to a consistent schedule - irregular lighting actually encourages algae more than consistent long periods.
UV steriliser: A UV steriliser fitted in-line with your filter kills free-floating algae cells (and bacteria) as water passes through. It's the fastest fix - most green water clears in two to three days. The downside is cost: a decent unit for a 200-litre tank runs £40-80 / $50-100. It won't fix the underlying nutrient issue, so combine it with reduced feeding and more frequent water changes.
Blackout: Three to four days of complete darkness (cover the tank entirely) will kill a green water bloom. Feed lightly or not at all during the blackout. Most plants survive three to four days without light; most algae won't.
What most guides miss: the stocking load problem
You can have perfect filtration technique and still fight chronic cloudiness if your tank is overstocked. More fish means more waste, more ammonia, more demand on biological filtration, and more dissolved organics that mechanical media can't touch.
Check your stocking level honestly. The old "1 inch of fish per gallon" rule is outdated and wrong for most setups - it doesn't account for fish body shape, waste output, or territorial needs. Use a proper stocking calculator to get a realistic picture of your tank's load. If you're overstocked, no amount of filter cleaning will give you crystal-clear water long-term.
The aquarium industry has a commercial interest in selling you more fish. Fish shops don't always tell you when a tank is full. If your water has been chronically cloudy for weeks despite good maintenance, overstocking is the most likely explanation. Adding more filtration helps, but removing fish helps more.
New tank cloudiness and the nitrogen cycle
If your tank is under six weeks old and the water looks milky, this is almost certainly a bacterial bloom associated with cycling. The tank's biological filter hasn't established enough beneficial bacteria to process waste yet, and free-floating heterotrophic bacteria temporarily dominate. It looks alarming but it's normal.
What not to do: large emergency water changes, replacing filter media, or adding chemical clarifiers repeatedly. All of these delay cycling. What to do: keep feeding lightly, maintain your normal water change schedule, and wait. The bloom typically clears within two to five days as the nitrogen cycle establishes.
If you haven't cycled your tank properly before adding fish, read our guide on how to cycle a fish tank - it'll save you a lot of grief. And remember: clear water in an uncycled tank can still have lethal ammonia levels. Test with a liquid test kit, not strips. See aquarium water parameters for target ranges.
Track your parameters and get care reminders with App-aquatic.
Get the free appMy tank water is clear but has a yellowish tint - is that a problem?
Usually not. Yellow-brown tinting is almost always tannins leaching from driftwood or certain substrates. It's harmless and some fish - blackwater species like Apistogramma or cardinal tetras - actually prefer it. If you want it gone, run activated carbon in your filter for 48 hours and it'll clear. Replace the carbon monthly if you want to keep it clear long-term.
How long does a bacterial bloom last in a new tank?
Typically two to five days, sometimes up to a week. It clears on its own as the tank cycles and bacterial populations stabilise. Don't do large water changes to try to clear it - you're diluting the bacteria the tank needs to establish. Keep feeding light and let it run its course.
Can I use tap water for water changes, or do I need RO water?
For most community fish, dechlorinated tap water is perfectly fine. Soft-water species like discus (Symphysodon spp.) or some South American dwarf cichlids do better with RO water or a tap-RO blend, but the average community tank with tetras, livebearers, and corydoras doesn't need it. Check your tap water's GH and KH - most suppliers publish this - and compare it to your fish's preferred range.
Does adding live plants help keep water clear?
Yes, meaningfully so. Live plants compete with algae for the same nutrients - nitrate, phosphate, CO2 - and a well-planted tank with fast-growing species like Egeria densa, Ceratophyllum demersum, or stem plants like Hygrophila polysperma will naturally suppress algae blooms. They won't fix a filtration problem, but they're a genuine long-term tool for water quality, not just decoration.
My water is clear but my fish look stressed - what am I missing?
Clear water can hide serious water quality problems. Ammonia and nitrite are both colourless and both lethal at low concentrations - 0.25 ppm ammonia is stressful for most fish; 1 ppm is dangerous. Nitrate above 40-60 ppm causes chronic stress even if it builds slowly. Test with a liquid test kit (API Master Test Kit is the reliable budget standard) and check your readings against species requirements. Don't diagnose by eye.
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