Diatoms (new-tank brown film)

Identification

Thin rust-brown, tan or golden-brown dusty film on sand, rock, glass and equipment, usually starting 1-4 weeks into a new tank. The film is powdery rather than slimy, feels gritty between fingers from the silica shells (frustules), has no strings, and wipes off glass easily. Coverage is usually under about 2 mm thick.

Often confused with

Dinoflagellates are snotty and stringy with bubbles and can be toxic to snails; diatom film is dry, dusty and harmless to grazers, and snails happily eat it. Cyanobacteria forms slimy sheets that peel and smear in red, green or purple. If snails are dying on a brown coating, suspect dinos and check under a microscope.

What it targets

Sandbed, rock, glass and equipment surfaces. Very heavy films can shade low corals and look unsightly, and background diatom growth in mature tanks feeds snails, copepods and other microfauna.

How it spreads

Diatoms are present in every system and bloom when dissolved silicate is available: fresh dry rock and sand, new glass and plastic, tap water, and worn DI resin are the usual sources, with phosphate and nitrate as supporting nutrients. The bloom consumes the available silicate and then fades.

Control (safest first)

Wait it out: most new-tank blooms fade on their own over weeks as silicate depletes and competitors mature. Patience is the primary control and chemicals are unnecessary. Use RO/DI water with fresh DI resin (0 TDS) for top-off and salt mixing so new silicate stops entering with every water change. Grazer clean-up crew: Cerith, Trochus, Astrea, Nerite and turbo snails all graze diatom film, and sand-sifting help keeps the bed turned over. Stock modest numbers the tank can support after the bloom fades. Light housekeeping: wipe glass, blow film off rock with a baster and siphon loose material during water changes for appearance while the phase runs its course. Avoid chemical fixes and aggressive stripping: removing a harmless phase with algaecides or zeroing nutrients can open the door to dinoflagellates, a far worse problem.

Natural predators

• Cerith snails→
• Trochus snails→
• Astrea snails→
• Nerite snails→
• Turbo snails→

Field notes

A new keeper on Reef2Reef reported diatoms lasting 6-8 weeks and nearly reaching for chemicals at week 5, then watched them fade within days around the 5-month mark after following the wait-it-out advice. A forum keeper added 12 turbo snails and reported rock nearly clean of diatoms in three days, while noting the snails left the sand alone, and other posters pointed to Cerith snails for working through the sandbed. Advice split on water changes during the bloom: one line of guidance holds off changes until the bloom nears die-off so fresh silicate does not feed a second round, while WetWebMedia guidance favors regular 20-25 percent weekly changes with aged saltwater in a cycling tank.