Cloudy water in the aquarium — what it means and what to do

Cloudy water is a symptom, not a disease. Under one label hide five different phenomena — each with its own cause, colour, and fix. The good news: in most cases cloudiness clears by itself, as long as you don't make it worse with nervous interventions.

This guide helps you identify the type of cloudiness by colour and character, then pick the right response: wait it out, change water, swap the filter, or run UV.

1. Quick diagnosis — what do you see

Start with identification. Each cloudiness type looks different and needs a different response.

2. White cloudiness — bacterial bloom

The most common type. The water looks as if you added a spoonful of milk — uniform, milky, no visible particles. Smell usually neutral.

When it shows up

• In the first 2 weeks of a new tank — normal during maturation.
• After adding a lot of fish at once.
• After the filter has been off for a long time (e.g. 12h+ outage).
• After replacing a large share of the filter media.
• After a fish dies and stays in the tank for a while.

What's happening

Heterotrophic bacteria (the ones that break down organics — not the nitrifiers from the cycle) explode in numbers, feeding on a spike in dissolved organic matter. They're so numerous you can see them as cloudiness. Once the fuel runs out, the population drops back and the water clears.

What to do

• Patience. A bloom runs 5–10 days and clears on its own. Interventions (big water changes, filter swaps) usually prolong it.
• Cut feeding by 50% until it clears.
• Do small water changes (15–20%) every 2 days — not big ones.
• Test NH₃ and NO₂⁻. If > 0, dose Seachem Prime every 24 h.
• If the bloom lasts > 14 days — hunt for a hidden dead fish or rotting matter under the substrate.

What NOT to do

• Replace filter sponges (you'll kill the cycle and make it worse).
• Do a 100% water change (a reset that extends the bloom).
• Add "water clarifier" without diagnosis — often masks the problem.
• Dose bacterial starters (don't help with heterotrophs).

If you're not yet comfortable with the nitrogen cycle — see our nitrogen cycle guide. Bacterial blooms often start right at that stage.

3. Green water (Euglena bloom)

Looks like: water as green as pea soup, visibility drops to a few centimetres. Not bacteria — single-cell algae (usually Euglena) suspended in the water column.

Cause

Excess light + nitrate + phosphate. Classic triggers: missed feeding pulled out late, an unnoticed dead fish, or a photoperiod > 10 h.

Fix (fastest → slowest)

1. UV sterilizer (5–9 W for 100 L) — 24–48 h and water is crystal clear. The most effective.
2. Blackout 3–4 days — completely cover the tank. Euglena dies without light.
3. Fine filter pad + patience (several days).
4. After clearing: big water change, cut photoperiod to 6–7 h, test NO₃ and PO₄.

More in our algae guide — green water is covered there as one of 9 algae types.

4. Brown / yellow water — tannins

Looks like: water the colour of brewed tea — from pale yellow to dark brown. Transparent, you can see through, but it's "coloured".

Cause

Tannins, humic and fulvic acids released from:

• Driftwood (mopani, mangrove, catappa) — most common.
• Indian almond (catappa) leaves — deliberately added to Amazon biotope tanks.
• Peat in the filter.
• Some active substrates (ADA Amazonia, especially in the first months).

Is it a problem?

No — often a benefit. Tannins:

• Lower pH (good for Amazon fish and Caridina shrimp).
• Have antibacterial and antifungal properties.
• Reduce fish stress (mimic the natural blackwater environment).
• Help with fry incubation.

In blackwater biotopes (discus, apistogramma, wild bettas) tank colour is deliberately maintained.

How to remove it if you don't like it

• Activated carbon in the filter for 1–2 weeks. Removes tannins in 5–7 days.
• Seachem Purigen — synthetic resin, stronger than carbon, reusable after regeneration.
• Pre-soak driftwood in a bucket for 1–2 weeks before adding (change water daily).
• Frequent water changes (30% weekly) gradually dilute tannins.

Note: removing tannins raises pH. If fish are acclimated to lower pH — do it gradually.

5. Grey / dusty cloudiness

Looks like: grey or tan "clouds" in the water, usually settling within 2–24 hours. Can look like swirling dust.

Common causes

• New sand or gravel not rinsed thoroughly — dust lifts as you fill.
• Pouring water from a height — impact on the substrate lifts fine particles.
• Rooting fish — goldfish, large cichlids, loaches. Constant substrate disturbance.
• Activated carbon dust on day one (rinse carbon before use).
• New filter media dust (ceramic, bio-balls) — same story.

Fix

• In 90% of cases it clears by itself within 2–24 h. The filter will catch it.
• Polishing pad (Seachem Polishing Pad, fine floss) speeds it up.
• Pour new water slowly onto a filter lid, a small plate or into your palm — minimises stirring.
• For tanks with rooting fish — permanent coarser mechanical media + weekly squeeze.

6. White "streaks" / micro-bubbles

Looks like: microscopic air bubbles suspended in the water. The tank looks "fizzy" or has a characteristic shimmer.

Common causes

• Filter outlet above the water surface — turbulence sucks in air.
• Leaky canister filter — micro-leaks suck in air at the intake. Classic symptom of worn seals.
• CO₂ diffuser with an air leak (a micro-gap at the hose connection).
• Pouring water from height during a change — temporary, clears in 2–4 hours.

Fix

• Lower the filter outlet so the surface barely ripples — no loud splash.
• Check seals on the canister filter — if it's 3+ years old, worth replacing.
• Check every CO₂ hose connection — nuts, washers, the diffuser itself.
• After a water change: bubbles usually clear on their own within 2–4 hours.

7. Cloudiness after a water change

Light cloudiness for 2–6 hours after a big change is normal. Usually it's a mix of:

• Tiny temperature differences (water density briefly layers — the "schlieren" effect).
• Substrate stirred up while refilling.
• Micro-bubbles from the pouring process.

When is it a problem?

• If cloudiness persists over 24 hours — you've kicked off a bacterial bloom or stirred the substrate too hard. Test NH₃.
• If the water turned green — Euglena. See section 3.
• If it smells like mud — possible BGA kicked up from the substrate. Measure, observe for a day.

8. Cloudiness that won't clear — action plan

If 10+ days have passed and the water is still cloudy, work through this in order:

1. Full parameter panel: NH₃, NO₂⁻, NO₃⁻, pH, KH, GH.
2. If NH₃ or NO₂⁻ > 0 — classic bacterial bloom from overload. 30% water changes every 2 days, halve feeding, Seachem Prime. Time: 7–14 days.
3. If parameters are fine and water is white — patience, don't do anything drastic. Hunt for hidden rotting biomass (dead fish, rotting plant, buried food).
4. If water is greenish — green water. UV filter or blackout.
5. If water is brown and bothers you — activated carbon in the filter. Clears in a week.
6. If none of the above — check the filter (running, not clogged), rinse in tank water, possibly swap the mechanical floss.

9. Prevention

Habits that minimise cloudiness:

• Don't overfeed. Once a day, only what fish finish in 2 minutes. Leftovers = fuel for bacterial blooms.
• Rinse new substrate (sand, gravel) until the runoff is clear. 10–20 minutes of work.
• Soak driftwood 1–2 weeks before adding (change the water every 2 days).
• Pour water slowly — onto a filter lid, a plate or into your palm.
• Rinse activated carbon and new filter media before adding.
• Good mechanical filtration: coarse + medium + fine pad. Squeeze the fine pad regularly.
• Vacuum the substrate where food collects.
• Check canister filter seals every six months.

10. FAQ

Does cloudy water hurt fish?

Bacterial blooms usually don't harm fish directly — but during a bloom dissolved oxygen drops. If fish gasp at the surface or act restless, add extra aeration. BGA and high NH₃ are separate threats (see parameters).

How long does a bacterial bloom last?

Typically 5–10 days. Over 14 days means the bacteria have a continuous fuel source — check if something's rotting (dead fish, food buried in substrate).

Can I use a water clarifier?

"Water clarifier" products (JBL Clynol, Seachem Clarity) bind tiny particles into larger clumps the filter catches. They work on grey/dusty cloudiness. On bacterial blooms — only partially, because individual bacteria are ten times smaller than the clarifier can bind. As a one-off — fine; as a routine — no.

Why does every water change leave it cloudy?

If 2–6 h — normal, it clears. If every time it takes longer — you're probably stirring the substrate too hard while refilling. Pour slowly, onto a filter lid. Alternatively: your changes are too big with unaged tap water poured fast.

Does UV clear every cloudiness?

UV effectively destroys: bacterial blooms (24–48 h), green water (24 h), some parasites. UV does NOT help with: mechanical cloudiness (sand, silt), tannins, micro-bubbles. Match the method to the cause.

Will more filtration help?

Mechanical filtration (floss, polishing pad) — yes, it catches fine particles. Extra biological filtration — yes, provided the media is seeded (a sponge from a mature tank). A fresh, uncycled filter won't speed anything up.

Is brown water a problem for plants?

It can be — dark water reduces the light reaching plants. In blackwater biotope tanks, choose low-light-tolerant species (Anubias, Microsorum, Cryptocoryne). In high-tech planted tanks, avoid heavy staining.

Wrap-up

Cloudy water usually looks worse than it is. Identifying by colour covers 80% of diagnosis — white = bacterial bloom, grey = stirred substrate, green = Euglena, brown = tannins, bubbles = filter. Each type has the right response, and the wrong one (panicked big changes, chemicals) usually makes it worse.

General rule: if parameters are fine, patience beats intervention. Filter and bacteria will solve 80% of issues on their own — just don't get in their way.