Diatoms: why a new aquarium turns brown
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
A few weeks after filling a new aquarium, a brownish, dusty film often appears on the glass, decor and substrate: these are diatoms, micro-algae that use silicate as a growth nutrient. A reference study in plankton ecology showed that silicate availability directly determines diatom dominance in an aquatic environment, at the expense of other micro-organisms (original study). Find our other equipment pointers in the accessories section.
Contents · 5 sections
Where the silicate in a brand-new tank comes from
Sand and some technical substrates naturally leach silicate in the first weeks, as does tap water in some regions. This high availability, combined with the lack of competition from other algae not yet established, creates ideal conditions for a quick diatom bloom, often before plants have even settled in.
A normal phase, not a failed start
Unlike green algae, which often signal excess light or nutrients built up over time, diatoms in a new tank are transient: available silicate is gradually depleted and the bloom generally fades within a few weeks, unrelated to a cycling mistake. Our guide to cloudy water in a new aquarium covers another equally common phase at this stage.
How to manage the phase
Regular manual glass cleaning, an algae-grazing fish or snail, and a moderate lighting duration limit the extent of the film without stopping it entirely, since silicate is not affected by light the way green algae are. We cover the right lighting settings in lighting duration and algae.
When to actually worry
A diatom bloom that persists beyond two months in a tank that is already planted and cycled deserves a water parameter check rather than repeated cleaning alone: it may then signal an ongoing imbalance rather than a simple start-up phase.
Frequently asked questions
Should I treat diatoms with an anti-algae product?
Rarely necessary: regular mechanical cleaning is enough in the vast majority of cases, as the phase stops on its own.
Are diatoms dangerous to fish?
No, they are harmless to fish; the main downside stays cosmetic, on the glass and decor.
Can their disappearance be sped up?
A grazing snail or fish helps limit the visible film, but full disappearance mostly depends on the natural depletion of available silicate.
See it in the Planète Pets shop: Stick-on digital thermometer for aquarium and terrarium glass (€18.90) · 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.
