I bought a 40-liter aquarium a week ago, filled it with water, added substrate and a few fish. Now the water has turned white and cloudy, even though I'm trying to keep everything clean. Is this normal or is something wrong with my equipment?
It's actually completely normal and has nothing to do with cleanliness. That cloudiness in a new tank is almost always a bacterial "bloom" - bacteria that are actively multiplying in the fresh water. The substrate and decorations release nutrients, and bacteria love that. It's called a "bacterial explosion" and it means the tank is going through the natural process of establishing biological balance.
The main thing is don't panic and change anything. You don't need to clean everything, you don't need to do a full water change, and you don't need to buy any special products. Usually the cloudiness clears up on its own in a week or two, when the bacteria reach equilibrium and start limiting each other. Just make sure your filter is running - that'll help the process move faster. Try not to overfeed your fish, because leftover food just makes things worse. If the water gets really white and you can't see anything, you can do a partial water change with 20-30 liters of fresh water, but it's not necessary.
Next time you set up a new tank, it helps to do a "seeded" startup - you can ask friends or a pet store for a bit of substrate or water from an established tank. That speeds things up. But for now, just wait and watch - everything will sort itself out.
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