Diatoms

Brown unicellular algae that appear in new aquariums. They form a brown film on glass, substrate and decorations. They disappear on their own as they mature.

Detailed explanation

Diatoms are single-celled algae that build a shell of silica (silicon dioxide). They almost always appear in the first 2-3 months of an aquarium's life, whether marine or freshwater. The brown veil that covers everything is their unmistakable sign.

Their appearance is linked to the availability of silicates in the water and the lack of competition with other organisms. In a new aquarium the silicates come from the substrate (sand, rocks), from the tap water and even from the silicone in the joints.

Diatoms are harmless. They do not harm fish or corals. They are ugly to look at but are part of the natural maturation process of the aquarium.

The best solution is patience. As the aquarium matures the silicates are consumed and the diatoms disappear on their own within 4-8 weeks. Meanwhile, algae eaters (Neritina snails, Otocinclus in freshwater aquariums, Trochus and Astrea snails in marine aquariums) actively consume them.

To prevent them or reduce their extent you can use osmosis water (which does not contain silicates) and antisilicate resins in the filter.

In marine aquaria, diatoms sometimes return to mature aquariums after a disturbance: a large change in substrate, the addition of new rocks, or a silicate spike detected by ICP.

Don't confuse diatoms with cyanobacteria. Cyanos form viscous and smelly mats. Diatoms form a dusty film that can be easily removed with a cloth.

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