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Why Your Aquarium Is Cloudy — Five Causes and What Each One Means
Cloudy water is the most common thing people bring to the counter, and it is the least specific. Cloudiness is a symptom with at least five different causes, and the fix for one of them makes another worse. Before you add anything to the tank, work out which one you have. Three questions get you most of the way: what colour is the haze, how quickly did it appear, and what changed in the week before it started?
1. Bacterial bloom — white or milky, appears over a day or two
By far the most common cause in a new or recently disturbed tank. Free-floating heterotrophic bacteria multiply on a sudden surplus of dissolved organics, and there are enough of them to scatter light. It typically follows a new setup, a big media change, a filter that was off for several hours, or an overfeed.
The counter-intuitive part: this one usually clears itself in three to seven days. Large water changes remove some bacteria but also some nutrients, so they slow the crash rather than speed it. What helps is stopping the surplus at source — feed lightly for a few days, siphon out any uneaten food and mulm, and leave the filter alone. What does not help is adding a clarifier, which clumps the bacteria into particles your mechanical media then has to catch.
Check ammonia while it is happening. A bloom with zero ammonia is cosmetic. A bloom with measurable ammonia means the biological colony has been damaged and needs support.
2. Fine particulate — grey or brown, appears immediately
Sand or gravel dust after a substrate change, or detritus stirred up by rescaping or a vigorous gravel vacuum. It appears within minutes rather than days, and it settles if you switch the filter off for an hour.
This is a purely mechanical problem and it wants a mechanical answer: a fine floss or polishing pad in the filter, added downstream of the coarse sponge. It will clear within a few hours and the pad will need rinsing or replacing afterwards. New substrate should be rinsed before it goes in — repeatedly, until the rinse water runs clear.
3. Green water — unmistakably green, builds over weeks
Free-floating algae, driven by light and nutrients. The usual culprits are a tank in direct sun, a light left on twelve or more hours a day, or nitrate and phosphate accumulating from overfeeding and infrequent water changes.
Water changes alone rarely fix it, because the algae reproduce faster than you dilute them. What works is removing the driver: cut the photoperiod to six or seven hours, move the tank out of direct sunlight, and reduce the nutrient load. Where the bloom is entrenched, a UV clarifier in the flow path will clear it in a few days — our internal and submersible filters include models with UV built in. Treat the UV as the tool that buys you time while you fix the underlying cause.
4. Tannins — yellow or tea-coloured, water is clear but tinted
Strictly speaking this is discolouration rather than cloudiness: the water stays transparent, it just has a colour. It comes from driftwood, botanicals and some substrates leaching tannins.
It is harmless — mildly beneficial for blackwater species, in fact — so the only question is whether you like the look. If not, activated carbon removes it within a day or two, and new driftwood can be soaked or boiled before it goes in the tank to get the worst of it out first.
5. A filter that has stopped doing its job
If the haze has crept in gradually with no obvious trigger, look at the filter before you look at the water. A clogged sponge cuts flow to a trickle. A worn impeller or an air-locked canister does the same. A unit undersized for the current stocking level was never going to keep up in the first place.
Check actual flow at the outlet rather than trusting the box rating. If it is well down on what it was, strip and rinse the mechanical media in old tank water, inspect the impeller and shaft for wear, and check the intake for a snail or a leaf. If the tank has been restocked since the filter was chosen, the filter may simply be too small now — our approach to sizing and the ratings on every listing will tell you quickly.
Working through it in order
- Test ammonia, nitrite and nitrate before doing anything. The readings separate a cosmetic problem from a dangerous one.
- Name the colour. White, grey, green and yellow point to different causes.
- Think back a week. New substrate, new fish, a filter service or a holiday feeder will usually be the trigger.
- Fix the cause, not the symptom. Clarifiers and chemicals treat appearance; they do not stop the process producing the haze.
What not to do
Do not tear the filter down and replace all the media at once — that turns a cosmetic bloom into a genuine cycling problem. Do not carry out a ninety per cent water change in a panic; large sudden swings in temperature and chemistry stress fish more than the haze does. And do not stack three different treatments in the same week, because you will not know which one worked.
If you have worked through the list and are still stuck, get in touch with your test results and a description of the haze. Common questions are answered on our FAQ page, and the full filtration range is here if the diagnosis turns out to be an undersized unit.
From our shelves
Products in this store that relate to what is covered above:
Prices are in US dollars. Delivery timings are on our shipping policy page and the return and refund policy sets out the window if something is not right.
