Why New Tank Glass Gets Brown Algae (Diatoms) — and When It Clears

Written 2026-10-10

The brown film on new tank glass and substrate is diatom algae. It feeds on silica released by fresh substrate and tap water, and unlike green algae, it grows fine under weak light too. Running the Water Kiwoogi simulator on a 60cm standard tank with plain black sand and no livestock, diatoms show up by day 6 and are still growing at day 60. Add six neon tetras and a bacteria starter to the same sand, though, and diatoms never even reach the visible level (15) before clearing completely by day 35.

It feeds on silica, and barely needs light

Diatoms eat dissolved silica and form a thin brown film on glass, substrate, and plant leaves. New substrate and tap water both release silica, and right after setup there’s nothing else — no other algae, no plants — competing for it. Green algae need strong light to spread, but diatoms grow as long as the light is on at all, regardless of intensity. That’s usually what’s behind “algae even with a weak light.”

The substrate decides how much shows up

Comparing substrates on an empty, light-only 60cm standard tank: a bare bottom runs out of silica fast, since it only has what the tap water carries, and clears on its own, while black and white sand keep releasing silica for weeks and the diatoms still haven’t cleared by day 60.

A stocked, fed tank clears faster than an empty one

Counterintuitively, the “cleanest” tank — light on, no fish, no food — is the one where diatoms stick around longest. What pushes diatoms out is a mature biofilm, and biofilm grows by eating organic matter from fish waste and leftover food. With no livestock there’s no organic matter, so biofilm barely develops, and nothing holds the diatoms back as long as silica remains. Add six neon tetras to the same black sand tank, feed them, and throw in a bacteria starter, and the faster biofilm growth keeps diatoms from ever reaching the visible threshold (15) — they peak at 10 and are gone by day 35. All six fish were still alive at day 60.

What to actually do

If you’re feeding fish normally, brown algae on the glass isn’t something to worry about — it clears on its own within weeks as biofilm builds up. The harder case is a tank with no livestock yet, or a plant-only display. Since silica can keep it going, scraping the glass and doing water changes is the fallback — though tap water itself carries silica (we didn’t run this exact case), so frequent water changes also resupply some of it. Algae-eating species like otocinclus catfish or nerite snails are another option; their efficiency varies a lot by species, so we’ll cover that comparison in a separate guide.

The button below is the 60cm standard tank · 4cm black sand · six neon tetras + bacteria starter · 60 days tank from this guide.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.