Cloudy aquarium water: which kind of cloudy you're looking at

Cloudy water looks more dramatic than it usually is. Almost always one of five causes is behind it, and most of them clear on their own if you give the tank a few quiet days. The hard part isn't the fix, it's telling them apart: a milky bacterial bloom, stirred-up substrate, an algae bloom and precipitating calcium look alike at first glance but call for nearly opposite responses. Scrub the filter and change half the water every day during a bacterial bloom and you prolong it. Wait out green water and you'll wait a long time.

Before you do anything: fill a glass with tank water and hold it against a sheet of white paper. Let it stand for an hour. Whatever has settled at the bottom was suspended particles. Whatever stays evenly white or grey is bacteria or calcium. Whatever stays green is algae. That one look saves you most of the wrong moves.

The five usual causes, most common first

1. Milky white in a new tank: a bacterial bloom

This is the classic of the first weeks. A freshly set-up tank holds plenty of dissolved organics — from the substrate, from plant trimmings, from the first meals — but hardly any bacterial community to keep them in check yet. Free-floating bacteria seize the opportunity and multiply until the water turns milky white to grey. The same bloom can hit a cycled tank after an over-thorough filter clean, a lot of new material, or heavy overfeeding.

A bacterial bloom is usually harmless to fish as long as there's enough oxygen — the bacteria use some themselves. It generally clears on its own within a few days to two weeks, once the nutrients run out and the bacteria settled in the filter catch up. What matters most is what you don't do: don't clean the filter, don't change large amounts of water every day, don't pour in clarifiers. Feed sparingly, keep the surface moving, and give the tank time.

Keep an eye on NH₄ and NO₂ during this phase. If they climb, the cloudiness isn't the problem — the nitrogen cycle that isn't carrying the load yet is, and the rules of cycling apply, not the rules of cloudy water.

2. Grey and gritty right after setup: stirred-up substrate

Right after filling, after a rescape or after a vigorous gravel vacuum, fine dust from gravel, sand or soil hangs in the water. It's the only kind of cloudiness that settles in the glass. It usually clears within hours to two days, faster with a layer of fine filter floss on top of the filter media that you take out again afterwards. Next time you fill the tank, pour onto a plate or a plastic bag laid on the substrate.

3. Green water: an algae bloom

When the water turns green rather than white, single-celled algae are floating in it. They need light and nutrients: long photoperiods, direct sunlight on the tank and raised nitrate and phosphate are the usual combination. Unlike a bacterial bloom, green water rarely clears on its own. Water changes alone usually don't do it either, because the algae multiply faster than you dilute them.

What reliably works: a UV clarifier that kills the floating algae, or a blackout of three to four days with the tank covered — plants survive it, the algae don't. Then go after the cause: shorten the light, keep sunlight off the tank, feed less, and bring nitrate and phosphate into a range the plants actually take up. Why the ratio of the two matters more than either number alone is in the guide to the nitrate-phosphate ratio.

4. Yellowish to brown, but clear: tannins

Water that looks like weak tea isn't cloudy, it's tinted. Driftwood, leaves such as catappa and peat release humic substances and tannins. That's harmless — welcome, even, for many soft-water fish — and lowers the pH slightly. If the colour bothers you, activated carbon in the filter and regular water changes remove it bit by bit. Soaking new wood before it goes in shortens the phase.

5. A white haze after hardening or dosing: precipitating calcium

If the water turns milky right after adding remineralising salts or KH powder, or after a water change with very hard tap water, calcium carbonate is precipitating. It happens when KH, calcium and pH all run too high at once. Reef keepers know the same thing as a snowstorm, when calcium and alkalinity supplements are dosed too fast or in the same spot. The haze usually settles within a day. In future, dissolve supplements first, add them slowly where the flow is strong, and don't dose calcium and KH at the same time.

Same haze, different warning

If the water smells foul, the cloudiness is grey-white and leftover food or a dead animal is involved, the cloudiness is a symptom of decay. Then clarity isn't what counts — ammonia is: test NH₄ right away. Anything above 0.1 mg/L doesn't belong in the tank; above 0.25 mg/L it becomes acutely dangerous for fish. What to do in that case is laid out step by step in the guide to ammonia in a cycled tank.

If fish are gasping at the surface, that's an alarm whatever the cause. A dense bacterial bloom eats oxygen — add surface movement immediately, with an air stone or a raised outlet.

Your playbook, step by step

1. Do the glass test. If it settles, it's dust. If it stays white, more likely bacteria or calcium. If it stays green, algae. Clear but tinted, tannins.
2. Test NH₄ and NO₂. If both read zero, the cloudiness is cosmetic. If either is above, deal with the nitrogen cycle first.
3. Think back over the last few days. New tank, deep-cleaned filter, new wood, hardening salts, more food than usual? The cause is almost always whatever changed last.
4. Leave the filter alone. In a bacterial bloom it needs its bacteria; for every other cause, cleaning doesn't help.
5. Feed sparingly and aerate. It helps with every one of the five causes and hurts none.
6. Treat green water on purpose. UV clarifier or blackout, then fix the light and the nutrients.
7. Be patient with white. A bacterial bloom still there after two weeks is rare — left alone, it's usually gone in a few days.

What reefnotes does with cloudy water

Cloudiness isn't a reading, but it belongs in the logbook. Under Care → Other you log an observation with a date and a note, like "milky since Monday" or "green after the sunny day". It then sits next to your tests and water changes, and two weeks later you can see whether the cloudiness lined up with an event.

The values that place a cloudy spell, reefnotes reads the way that fits the cause. NH₄ and NO₂ are signs of whether the cycle is carrying the load, not values to dose up — the app never suggests a product for them; it shows you when they're back at zero. Nitrate and phosphate, on the other hand, are nutrients, and their trend during green water tells you whether the plants keep up with the supply. How a new tank is expected to develop over its first weeks is sketched by the nitrogen cycle timeline tool, and the guide to the first 30 days of a freshwater tank walks you through exactly the phase in which most cloudy water appears. A cloudy tank is rarely a sick tank. Most of the time it's one that's sorting itself out.