Summary: Aquarium water that turns white, yellow, or foul-smelling is mostly related to organic matter accumulation, bacterial growth, and increased suspended particles. First use a water quality test pen to measure turbidity and organic matter, figure out which type of problem it is, and then treat it accordingly—this is more reliable than blindly doing large water changes.
Keywords: what to do when aquarium water turns white, aquarium water turns yellow, aquarium water smells foul, aquarium water is cloudy, causes of aquarium water quality deterioration
One-sentence answer: Whitening is mostly suspended particles or a bacterial bloom—check turbidity; yellowing is mostly accumulation of dissolved organic matter—check DOM and UV254; foul smell is mostly too much organic matter and oxygen-consuming decomposition—check BOD, COD, and TOC. The Water Detective Aquarium Edition can measure all these indicators at once, and the AI in the mini-program will tell you roughly what type of problem it is and what to do first.
Which indicators to check for common phenomena
| Phenomenon | Common cause | Key indicators | What to do first |
|---|---|---|---|
| White, cloudy | New tank not properly cycled, overfeeding, filter media buildup, substrate disturbed | Turbidity | Reduce feeding, check filtration, small water change |
| Yellowing | Dissolved organic matter from decomposition of driftwood, dead leaves and fish feces | DOM, UV254, TOC | Add activated carbon, clean up residue, change water in stages |
| Foul odor, fishy smell | Too much organic matter, dead fish or leftover food rotting | BOD, COD, TOC | First find and remove the rotting source, increase aeration |
| Algae growth, tank walls turning green | Excess organic nutrients, excessive lighting duration | COD, TOC | Reduce lighting and feeding |
| Fish gasping at the surface, listless | Organic matter decomposition competing for oxygen, sudden water quality changes | BOD, COD, water temperature | Increase oxygen immediately, change a small amount of water, and use test reagents to measure ammonia nitrogen |
Why not do a large water change as soon as you notice it
Changing too much water at once causes sudden changes in water temperature and water quality, which makes fish more prone to stress, and the beneficial bacteria in the filter media may also be affected. First test to identify the type of problem, then change water in portions according to 20%-30%, and test again after each change to see whether the readings are moving back.
Organic matter is the "source"
Rising ammonia nitrogen and nitrite are often the result of organic matter decomposition. By keeping an eye on organic matter first, you can act early, before the water discolors or the fish show problems. The TOC, COD, BOD, DOM and UV254 measured by the Water Detective Aquarium Edition look at the amount and types of organic matter from different angles.
Frequently Asked Questions
Q: Is it normal for the water in a new tank to turn cloudy white? In the first few weeks of a new tank, the bacterial balance has not yet been established, so cloudy white water is common. You can measure turbidity every day; if you see it trending downward, that means the tank is becoming more stable.
Q: The water looks very clear, but the fish keep dying? Clear water does not mean good water. Dissolved organic matter is invisible to the naked eye. You can measure BOD, COD and UV254, and at the same time use test reagents to measure ammonia nitrogen and nitrite.