Why Is My Aquarium Water Cloudy: White Green Yellow
Cloudy aquarium water has three distinct colour fingerprints, and the colour points directly to the cause: white-grey indicates bacterial bloom, green indicates suspended algae, yellow-brown indicates tannins or diatom dust. Why is my aquarium water cloudy — the answer depends entirely on the colour, and treating one as the other wastes weeks. This guide from Gensou Aquascaping at 5 Everton Park walks through nine diagnostic branches so you can identify the cloud type within minutes and select the correct intervention.
Cause 1: White-Grey Bacterial Bloom
White-grey haze that develops two to seven days after a new tank fills indicates a heterotrophic bacterial bloom feeding on dissolved organic compounds. The bacteria are harmless and the bloom typically clears in five to fourteen days as nutrients deplete. Do not perform large water changes — this prolongs the bloom by replenishing nutrients. Reduce feeding by 50 per cent, ensure good surface agitation, and wait. Bacterial blooms are a normal cycling phase.
Cause 2: Green Water (Suspended Algae)
Pea-soup green water is single-celled phytoplankton suspended throughout the water column, triggered by excess light and nitrate. Reduce photoperiod to 6 hours, perform a 50 per cent water change, and run a UV steriliser (SGD 80-180 from the aquarium equipment range) for 5-7 days. Diatomite filter floss in the canister also clears it within 48 hours. Long-term: control nitrate below 30 ppm and reduce light intensity by 25 per cent.
Cause 3: Yellow Tannin Stain
Yellow-amber water in tanks with driftwood, Indian Almond leaves or alder cones is tannin staining and is not actually cloudy — it is clear but coloured. This is desirable in blackwater biotopes (bettas, discus, apistogramma) and cosmetic in others. Remove if undesired by running activated carbon in the filter for 1-2 weeks, or by leaching new wood in a separate bucket for 7-14 days before adding to the tank.
Cause 4: Brown Diatom Bloom
Brown dust on glass and decor in new tanks is diatoms feeding on silicates leaching from new substrate and tap water. The bloom peaks at three to six weeks and resolves naturally as silicates deplete. Wipe glass weekly with a magnet cleaner from the aquascaping tools range and add otocinclus or nerite snails for biological cleanup. Do not medicate — diatoms are harmless and self-limiting.
Cause 5: Substrate Dust from Recent Disturbance
Cloudy water within hours of substrate disturbance (rescaping, new substrate addition, vigorous gravel vacuum) is fine particulate suspended in the water column. Run filter floss or Polyfilter in the canister and the cloud clears within 6-24 hours. Use a pre-rinsed substrate from the substrate range and add it slowly under a plastic bag to minimise disturbance during initial setup.
Cause 6: Overstocking and Overfeeding Cloud
Persistent low-grade haze in established tanks indicates dissolved organic compound (DOC) overload from overfeeding and/or overstocking. Test ammonia and nitrite first to rule out cycle crash, then reduce feeding by 30 per cent, increase water-change frequency to 25 per cent weekly, and consider adding mechanical filtration with finer pad. The cloud clears within 7-14 days of management changes. Long-term: reduce stocking or upgrade filtration.
Cause 7: New Tank Cycling Cloud
Cycling tanks routinely produce intermittent cloudiness as bacterial populations boom and bust on shifting nutrient sources. White haze, brief green tint, and intermittent clear all alternate during weeks 2-6 of fishless cycling. Continue dosing ammonia source per fishless cycle protocol, test ammonia and nitrite weekly, and resist the urge to perform large water changes which only delay the cycle. Cloudiness resolves once the cycle completes.
Cause 8: Filter Failure or Bypass
A filter that has stalled, has bypass flow around clogged media, or has been newly cleaned with chlorinated tap water produces persistent haze because mechanical filtration has dropped. Check the filter outflow visually for vigorous flow, rinse media in dechlorinated water (never tap), and verify intake strainers are clear. The water care range stocks bacterial supplements (Stability, Quick Start) to re-establish biological filtration after a clean.
Cause 9: Snail or Cherry Shrimp Boom Crash
Sudden mass die-off of snails or shrimp following parameter shock or copper exposure releases a cloudburst of decomposition organics that produce white haze and ammonia spike within 24 hours. Identify the dead population, remove all visible carcasses, perform a 50 per cent water change, dose Seachem Prime, and run filter floss. The cloud clears within 48-72 hours but ammonia must be monitored daily for a week.
Quick Identification Flowchart
Look at the cloud colour first. White-grey in a new tank is bacterial bloom — wait it out. Green is algae — UV steriliser plus light reduction. Yellow is tannin — carbon plus wood pre-soak. Brown dust on glass is diatoms — wait it out plus biological cleanup. White haze in established tanks is overfeeding — reduce feeding plus water change. Cloud after substrate disturbance clears with filter floss within 24 hours. The colour is the diagnosis — match the response to the colour.
Related Reading
emilynakatani
Still Have Questions About Your Tank?
Drop by Gensou Aquascaping — most walk-in questions get answered in under 10 minutes by someone who has set up hundreds of tanks.
5 Everton Park #01-34B, Singapore 080005 · Open daily 11am – 8pm
