Cloudy aquarium water — causes and fixes

DiagnosticWhite / milky haze. Tank is new (within 4 weeks) or has just been disturbed. Often coincides with elevated ammonia or nitrite. Doesn't respond to mechanical filtration.

FixWait. Heterotrophic bacteria are responding to a sudden nutrient pulse. Within 3–7 days the population stabilizes and water clears. If ammonia is also elevated, dose Dr. Tim's One & Only or Fritz TurboStart 900 to seed nitrifiers and accelerate the cycle.

DiagnosticGreen tint visible from the side. Worse near the lit end of the tank. pH may spike midday and crash at night (algae photosynthesis).

FixCut photoperiod to 6 hours, blackout 3 days, run a UV sterilizer (9 W per 50 gal at slow flow rate). Address the root cause: excess nutrients (test NO3, PO4) or new tank not yet stabilized.

DiagnosticJust rearranged sand, added new sand, or moved rock. Cloudiness is uniform throughout the tank, settles in 1–6 hours.

FixWait. Run a fine filter sock or polish pad to accelerate. If you're adding new sand, rinse it first (even 'pre-washed' aragonite). New CaribSea Arag-Alive is the exception — don't rinse, just dump.

DiagnosticMilky white cloud appears within minutes of dosing two-part calcium or alkalinity. Tank had been running fine.

FixStop dosing immediately. Test alkalinity AND calcium — if both are very high (alk >12 dKH AND Ca >480 ppm), you've crossed the saturation limit. Slow water changes over a week to bring parameters back in range. Going forward: never dose calcium and alkalinity simultaneously at the same spot in the tank.

DiagnosticYellow-brown tint, not gray/white. Smells faintly of tea. Common after adding new dry rock or wood not yet cured.

FixRun activated carbon (1 cup per 30 gal, replace weekly until clear). For driftwood, soak/boil first before placing in tank.

DiagnosticCloudy water immediately after dosing trace elements, amino acids, or coral foods. Specific to brands like Brightwell Aminomega, ESV, Red Sea Energy.

FixStop dosing the suspected supplement, do a 25% water change, run carbon. Re-introduce one supplement at a time at half the recommended dose.