1. New Tank or Sand Disturbance, White/Grey, Fine Particles

The most common cause of cloudiness in a new tank, and the least concerning.
When you first fill a reef tank or disturb the sandbed, fine aragonite particles
go into suspension and cloud the water. This isn’t harmful to anything and
clears on its own as the return pump runs and particles are either captured
by the filter media or settle back to the sandbed.

How to Identify It

• Appeared immediately or within hours of filling the tank or disturbing the sand
• Fine, white or grey milky appearance, not chunky or green
• No unusual smell
• Clears progressively over 24–48 hours as the pump runs
• Parameters test normal if the tank is already established

What to Do

• Run the return pump continuously, mechanical filtration captures the particles
• Do not do a water change to try to clear it faster, replacing water doesn’t remove suspended particles, and adding new water may re-disturb the sand
• Replace filter floss after the tank clears, it will be heavily loaded from capturing the particles
• If it hasn’t cleared within 72 hours of full pump operation, add a small amount of a flocculant product (like Two Little Fishies Acropower or a similar clarifier rated for reef tanks), this causes fine particles to clump and be captured by filtration more efficiently

Preventing It Next Time

Rinse sand under RODI water until the water runs clear before adding it to
the tank. When adding replacement water or doing a water change, pour slowly
onto a plate or piece of plastic placed on the sand surface, this disperses
the flow before it hits the substrate and prevents disturbance.

2. Bacterial Bloom, White/Milky, Often in New or Disrupted Tanks

A bacterial bloom is a sudden population explosion of free-floating bacteria
in the water column. It produces a white, milky cloudiness, sometimes
indistinguishable from sand disturbance at first glance, but with a different
cause and a different timeline.

Bacterial blooms happen when there’s a sudden change in available organic
material, a large food source becomes available and the bacterial population
responds by multiplying rapidly. Common triggers:

• New tank cycling: The decomposing ammonia source (raw shrimp) releases organic compounds that fuel a bacterial population burst in weeks 1–3 of the cycle. Expected and harmless during this stage.
• Adding a bottled bacteria product: Large doses of live bacteria dosed into a tank can produce temporary cloudiness as the bacteria population establishes. Clears within 24–48 hours.
• Overfeeding: A large uneaten feeding that reaches the sandbed provides the organic spike that triggers a bloom.
• Dead animal in the tank: The most serious trigger. A dead fish, invertebrate, or crashed coral releases a large organic load that fuels a rapid bacterial bloom, sometimes within hours.
• Large water change in a new tank: Changing the bacterial population balance can temporarily shift free-floating bacteria into the water column.

How to Identify It

• White or grey milky appearance, similar to sand disturbance but appeared without physical disturbance
• Appeared 12–48 hours after overfeeding, adding a new product, or an organic event in the tank
• Doesn’t clear within 24 hours the way sand disturbance does, persists for 2–5 days if source is ongoing
• May carry a faint organic smell if triggered by overfeeding or a dead animal
• Ammonia may be elevated if caused by a dead animal, test immediately

What to Do

1. Search the tank for a dead animal, this is the first priority if cloudiness appeared suddenly in an established tank. Check behind every rock, under every coral, in every corner. A dead fish behind the rockwork can be invisible from the front of the tank but still causing a significant organic load. Use your sense of smell, a dead animal produces a distinct sulfurous or acrid odor.
2. Test ammonia immediately, if a dead animal is fueling the bloom, ammonia will be elevated. If ammonia is above 0.25 ppm, do a 20% water change immediately.
3. Remove the dead animal or food source, the bloom clears on its own once the organic source is removed.
4. Run activated carbon, helps clear dissolved organics faster.
5. Reduce feeding for the next 5–7 days until the bloom clears.
6. Don’t do large water changes unless ammonia is elevated, water changes alone won’t clear a bacterial bloom faster than natural die-off once the organic source is removed.

Bacterial Bloom in a New Tank During Cycling

If your tank is in the first 2–4 weeks of cycling and the water turns milky
white, this is a normal cycling bacterial bloom. It doesn’t require intervention.
Continue running the return pump and testing parameters every 2–3 days. The
bloom clears as the bacterial population stabilizes.

3. Calcium/Alkalinity Precipitation, Sudden White Cloudiness After Dosing

This is the most chemically serious cause of white cloudiness and the one
that requires the fastest response. Calcium carbonate precipitation happens
when alkalinity and calcium concentrations are pushed beyond their solubility
limit simultaneously, the dissolved minerals spontaneously crystallize out
of solution, producing a sudden white cloud and crashing both alkalinity and
calcium simultaneously.

Triggers:

• Dosing alkalinity too fast, adding a large dose of sodium bicarbonate or two-part solution directly to the tank without dilution
• Dosing alkalinity into an area of very high calcium concentration (near the return nozzle where concentrated two-part drips in)
• Simultaneously high alkalinity AND high calcium, above 11 dKH alkalinity with calcium above 480 ppm creates precipitation risk
• Kalkwasser overdosed into the tank, kalk raises both pH and calcium rapidly and can trigger precipitation if the dose is too large

How to Identify It

• Appeared immediately or within minutes of adding a dosing product
• White, often milky appearance that may be more concentrated near where the product was added
• Test results will show both alkalinity AND calcium lower than before dosing, the precipitation has removed both from solution
• pH may have risen sharply then crashed as precipitation consumed buffering capacity

What to Do

1. Stop dosing immediately, do not add any more alkalinity, calcium, or kalkwasser.
2. Test alkalinity and calcium, determine how much both have dropped from the precipitation event.
3. Do NOT correct the parameter drop immediately, the tank needs to stabilize first. Dosing again immediately risks triggering further precipitation.
4. Run the return pump at full flow and allow water movement to gradually dissolve the precipitate back into solution over 24–48 hours. Some precipitation will be captured by mechanical filtration, that portion is permanently lost.
5. After 24 hours, test again and begin correcting alkalinity and calcium gradually, no more than 0.5 dKH per day for alkalinity, 10–15 ppm per day for calcium.
6. In future, always dilute dosing products in RODI water before adding to the tank, add them to a high-flow area away from other concentrated dose points, and never raise alkalinity or calcium more than 0.5 dKH or 20 ppm per day.

4. Phytoplankton or Algae Bloom, Green or Yellow-Green Cloudiness

A green or yellow-green cloudiness almost always indicates a phytoplankton
(single-celled algae) bloom in the water column. This happens when nutrients, nitrate and phosphate, are elevated and light is sufficient to drive rapid
algae reproduction in open water. The tank turns green rather than clear because
the free-floating algae population is large enough to color the water.

Green water is more common in:

• New tanks with elevated nutrients from cycling and no established filtration
• Established tanks that have been overfed for weeks without adequate export
• Tanks near windows receiving indirect sunlight
• Tanks with very long photoperiods (14+ hours)

How to Identify It

• Green or yellow-green tint to the water, sometimes subtle, sometimes pronounced
• Developed gradually over days to weeks, not suddenly
• Nitrate and/or phosphate testing elevated
• Tank may also show algae growth on glass and rockwork
• No unusual smell

What to Do

1. Reduce feeding volume, less food = less nutrient input = less algae fuel.
2. Reduce photoperiod, drop to 8–10 hours total if running longer. Light is the driver of phytoplankton growth; less light = slower bloom.
3. Check if any light source other than the tank light is reaching the water, a window nearby, a room light running 24 hours.
4. Run activated carbon, helps remove dissolved organics that feed the bloom.
5. Increase water change frequency, 10% twice per week instead of bi-weekly until nitrate and phosphate are back in range.
6. Replace filter floss immediately if it’s been more than 5 days since the last change.
7. Check skimmer output, a skimmer that’s not pulling skimmate is failing to export dissolved organics that feed the bloom.
8. Green water caused by nutrient imbalance clears within 1–3 weeks of addressing the above. UV sterilization can accelerate clearance if you have or can borrow a unit, it kills free-floating algae cells directly.

5. Protein Skimmer Overflow, White/Milky, After Water Change or New Addition

A protein skimmer that’s running too wet, or one that’s been disturbed by
a water change, a new coral addition, or a chemical change in the water, can overflow skimmate back into the sump or tank. When skimmate returns to
the water column, it produces a white, organic cloudiness that can look
alarming but is generally not dangerous in the short term.

This happens most commonly:

• Within 24–48 hours of a water change, new water has different surface tension and causes the skimmer to run wet temporarily
• After adding a new coral that releases mucus or terpenes into the water
• After adding a coral dip, medication, or other chemical that affects water surface tension
• When the skimmer cup overflows because it wasn’t emptied in time
• When the skimmer’s output valve has been set too wet and the collection cup fills and overflows

How to Identify It

• White cloudiness appeared after a water change, new addition, or product dosing
• Skimmer is producing a large volume of thin, watery, light-coloured foam rather than dark concentrated skimmate
• Skimmer collection cup may have overflowed visibly
• Cloudiness has an organic smell (like the skimmer cup contents)
• Clears within 24–48 hours once skimmer is adjusted

What to Do

1. Empty and clean the skimmer collection cup, clean the neck to remove salt creep that was partially blocking the foam from rising properly.
2. Raise the water level inside the skimmer body slightly by adjusting the output gate valve, this makes the skimmer run drier and stops the overflow.
3. If it occurred after a water change, this is normal. The skimmer will stabilize within 24–48 hours as the new water equilibrates. Reduce the skimmer output slightly for 24 hours if the overflow is severe.
4. Run activated carbon to clear the organic cloudiness faster.
5. Check the skimmer’s rated operating water depth, if the water level in the skimmer chamber is too high or too low relative to the manufacturer’s specification, the skimmer will not run correctly regardless of valve settings.

6. Dissolved Organic Yellowing, Not Cloudy, But Discolored

This isn’t true cloudiness, the water isn’t hazy, but it has a yellow
or amber tint that reduces clarity and affects how light reaches corals.
It’s caused by dissolved organic compounds (tannins, humic acids, and
breakdown products of organic material) that accumulate in reef water
over time and aren’t removed by biological or mechanical filtration.

Common contributors to yellowing:

• Leather corals releasing terpene compounds, one of the most significant sources
• Accumulated dissolved organics from overfeeding or insufficient skimming
• Infrequent water changes, dissolved organics build up over months without regular replacement
• New driftwood or natural materials added to the tank (not applicable to most reef tanks but occasionally seen)

What to Do

• Run activated carbon, the most effective single treatment for water yellowing. Replace with fresh carbon monthly. A tank with leather corals should run carbon continuously.
• Increase water change frequency temporarily, two consecutive 15% changes one week apart removes a significant portion of accumulated dissolved organics.
• Check skimmer output, a properly functioning skimmer removes dissolved organics continuously. Dark, concentrated skimmate is a sign it’s working.
• Yellowing resolves within 1–2 weeks of running fresh activated carbon in most cases.