Cloudy water in a new aquarium: the bacterial bloom explained
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
You have just set up your tank, the water was clear, and after a few days it turns whitish, milky, as if veiled. No panic: this is the famous start-up "bacterial bloom", a common and temporary phenomenon. It corresponds to an explosive growth of heterotrophic bacteria feeding on the tank's organic matter, before the useful nitrifying bacteria settle in for good. The gradual establishment of these bacterial populations in aquaria has been studied in detail, notably by Hovanec and DeLong in 1996. Find our other maintenance pointers in the aquarium care and hygiene section.
Contents · 4 sections
Why the water turns milky
In a new tank, the substrate, wood and any food release nutrients. Free-floating bacteria, quick to multiply, form a suspended cloud: that is what clouds the water. This haze is not an algae and is not fought like one; it simply reflects an ecosystem coming together. It usually clears on its own within a few days to two weeks.
What you must not do
The worst reaction is to empty and deep-clean the tank: you then restart the cycle from scratch. Also avoid massive water changes and cleaning the filter, which remove the bacteria settling in. Do not overdose food, and if fish are already present, feed very sparingly. Patience is your best ally here, as our guide to setting up a 60-litre aquarium step by step reminds you.
Telling a bloom from a real problem
A milky white haze at start-up is normal. Green water (algae), brown water (tannins or diatoms), or an established tank that suddenly clouds may signal something else: overstocking, overfeeding, a failing filter. Monitoring parameters then helps, as explained in our article on how many fish a 60-litre tank holds. A tank sized correctly from the start limits these imbalances: see our 60-litre aquarium starter kit.
Frequently asked questions
How long does a bacterial bloom last?
Generally from a few days to a fortnight. It resolves on its own once bacterial balance is reached.
Can I add fish during this cloudiness?
Better to wait until the tank is cycled and the water clear: adding fish too early exposes them to ammonia and nitrite spikes.
Does a "water clarifier" fix it?
These flocculants clump the particles but do not treat the cause. Letting the tank finish its cycle remains the lasting solution.
An accessory from our shop for this use: Hang-on isolation and breeding box for aquariums (€28.90) · the whole shop
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.
