Brown algae (diatoms): the new-tank dusting that clears itself

If your tank is a few weeks old and a dull brown coating has spread over the glass, the sand and the plant leaves — one that smears off easily when you wipe it — you almost certainly have diatoms. They look alarming on a fresh setup, but they are the most benign algae you will meet, and in most tanks they leave of their own accord.

Algae that build glass houses

Diatoms are single-celled algae with one remarkable feature: they build their cell walls out of silica — the same material as glass. Each cell lives inside an intricately patterned silica shell called a frustule, fitted together like a box and its lid. Building those shells means diatoms have a requirement no other aquarium alga shares: they need dissolved silicic acid in the water to grow (Martin-Jézéquel et al., 2000).

That single fact explains almost everything about the classic “new tank” brown algae. It also explains how to tell diatoms apart from other brown-ish problems: diatoms form a soft, dust-like film that wipes away with almost no effort, coating flat surfaces and settling on sand. If what you have is slimy, sheet-like and peels off in one piece — or smells musty — that is cyanobacteria, not diatoms, and it is dealt with quite differently.

A window that opens, then closes

A newly set-up aquarium is close to ideal for diatoms for a few weeks, because three things line up:

• Plenty of silica. New substrate, fresh rock and many tap-water supplies release dissolved silica into the water. A mature tank has drawn that down; a new one is flush with it.
• Little competition. The plants have not rooted in and grown yet, the microbial community is still establishing, and nutrient cycling is not yet stable — so diatoms face few rivals for light and nutrients. This is the same immaturity that a still-maturing filter shows from the bacterial side.
• Enough light. Diatoms are efficient at lower light than many green algae, so they can get going even before the tank is running at full brightness.

As the tank matures, that window closes. The readily available silica gets used up, the plants establish and start competing hard, and the microbial community settles — and diatoms, which are poor competitors once conditions stabilise, are pushed out, usually by green algae and by the plants themselves. In most tanks the brown phase runs somewhere from the second week to around the second month, then fades on its own.

Mostly, let the tank grow up

Because diatoms clear as the tank matures, the honest first-line treatment is patience plus a bit of housekeeping:

1. Wipe and siphon. Clean the glass with a pad or magnet and gently siphon the brown film off the sand and hardscape at water changes. It comes away easily — this is purely cosmetic upkeep while the phase passes.
2. Add diatom grazers. Once the tank is cycled, otocinclus catfish and nerite snails are outstanding at grazing diatom film off leaves and glass — few clean-up animals are as effective on this particular algae. (Otocinclus need an established tank with a steady supply of film to do well, so add them once the tank is ready, not on day one.)
3. Keep the plants growing and the light sensible. Anything that helps the plants establish faster shortens the diatom window; a healthy planted tank is what ultimately outcompetes them.
4. Don’t panic-dose. There is no need for chemical treatment or blackouts for a normal new-tank diatom phase — it is self-limiting, and reaching for algaecides risks more harm than the diatoms ever would.

When it isn’t just a new-tank phase

If a brown diatom film keeps returning in a tank that is well past its first couple of months, something is topping up the silica or holding the tank back. The usual culprits are a continuous silica source — silica-rich tap water, or RO water that is not fully stripping it — or a tank that is simply running with too little light and too few healthy plants to close the window. In that case the fix is the same as for most algae: get the plants growing well and the conditions stable. Persistent, genuinely stubborn diatoms are uncommon; a recurring film is worth investigating rather than fighting with chemicals.

1. Martin-Jézéquel, V., Hildebrand, M. & Brzezinski, M.A. (2000). Silicon metabolism in diatoms: implications for growth. Journal of Phycology, 36(5), 821–840.