If your brand-new aquarium turned milky-white a day or two after setup, relax — this is almost always a harmless bacterial bloom, a normal part of a new tank establishing itself. The worst thing you can do is panic and start big water changes.

Here is what is happening and what to do.

What causes it

A new tank has no established bacterial colony. Free-floating bacteria multiply rapidly on the organic matter in fresh water, turning it milky white within 24–48 hours. It looks alarming but is not dangerous.

If the cloudiness is grey and dusty rather than white, it is usually substrate dust from insufficiently rinsed gravel or sand — see our full cloudy water guide.

What to do

• Do NOT do big water changes — this feeds the bloom and prolongs it.
• Do NOT add clarifier products — they interfere with the developing bacteria.
• Wait. A bacterial bloom clears on its own within 5–14 days as the colony stabilises.
• If fish are already in the tank, feed lightly and test daily for ammonia — you are cycling with fish, which needs care. See our cycling guide.

Preventing it next time

Rinse substrate thoroughly before adding it, and cycle the tank fishlessly for 4–6 weeks before stocking. See how to set up a fish tank.

Frequently asked questions

Why did my new fish tank go cloudy after one day?

It is almost always a harmless bacterial bloom — free-floating bacteria multiplying in the fresh water of a new, uncycled tank. It clears itself within 5–14 days.

Should I do a water change if my new tank is cloudy?

No. Big water changes feed a bacterial bloom and prolong it. Wait it out. If fish are already in the tank, feed lightly and monitor ammonia daily.

How long does new tank cloudiness last?

A bacterial bloom typically clears within 5–14 days as the beneficial bacteria establish. Substrate-dust cloudiness usually clears within a couple of days of filtration.

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— Jaqline Smalling, FishForever