Diatoms (brown algae) in a new planted tank: why they come and go

What the brown film is

Diatoms are single-celled algae that, in the words of the Diatoms of North America project, live in houses made of glass: they are the only organisms with cell walls of transparent, opaline silica ^[3]. They photosynthesize with chlorophylls a and c ^[3], which gives a coating of them its golden to chocolate-brown color.

In an aquarium they form brown layers on everything: substrate, equipment, plant leaves, decoration, and the glass, where they can be stubborn in low-light tanks ^[1]. Aquasabi points out that the common name “brown algae” is wrong in the strict sense: true brown algae, the Phaeophyceae, are a separate group that, with a few exceptions, does not occur in fresh water ^[1].

Why they appear in new tanks

Aquasabi describes a typical timeline for a new planted tank. The first week usually stays clean; from the second week brown coats appear, followed by green thread algae and green films ^[2]. Its diatom page puts the brown phase at one to three weeks after set-up and calls it absolutely normal, needing no countermeasures ^[1].

Two conditions favor them at that stage:

• Silica in the water. Diatoms need dissolved silica to build their cell walls, and tap water with a high silicate level gives them material to multiply ^[1]. The US Geological Survey’s reference text on water chemistry gives the most common range of silica in natural water as 1 to about 30 mg/L, with up to 100 mg/L not unusual in some ground water ^[4]. It also notes that diatoms extract silica from the water they grow in, which visibly lowers it in some lakes and reservoirs ^[4].
• No competition. In a young tank there are few other algae and microorganisms to compete with, and newly planted stock is still adapting from its nursery form, so it grows slowly ^{[2, 1]}.

Our reading of the “they disappear by themselves” pattern (an inference, not a sourced finding): a new tank starts with a reservoir of silica from fresh tap water and a near-empty biofilm, which suits diatoms; as plants and green algae establish, the competition for light and surfaces shifts.

What clears them

• Wait, and wipe. Aquasabi expects green algae to crowd diatoms out within a few weeks of set-up, after which they usually do not return; meanwhile, siphon or wipe off the brown layers ^[1].
• Grazers. Aquasabi notes that algae-eating snails and shrimp readily eat diatoms ^[1], and Tropica’s treatment for algal films is a varied population of algae eaters plus a soft sponge on the glass ^[5]. Tropica ran a grazing test on tiles coated with green algae and diatoms; Amano shrimp were about twice as efficient as a Neritina snail and six times as efficient as a cherry shrimp ^[6].
• Plant mass. Planting densely from the start with fast-growing species, and adding floating plants or hornwort for a while, is Aquasabi’s general advice for the cycling phase ^[2]. Hornwort and floating plants have their own profiles.

If they will not go away

A diatom film that persists in an established tank with enough grazers is, according to Aquasabi, a reason to test silicate ^[1]. If it is high, the options are reverse-osmosis water, which contains only traces of silicate, or filter media that remove it. Aquasabi warns that those media also take out a lot of phosphate, which can harm plant growth ^[1]; if you use them, check phosphate and adjust dosing with the fertilizer dosing calculator.

A brown film that smells, comes off in slimy sheets and turns dark blue-green is more likely cyanobacteria.