Silicate (SiO₂)
The nutrient diatoms grow on. Usually a tap-water inheritance, usually responsible for new-tank brown algae.
What it is
Silicate is dissolved silica, present in almost all natural fresh water at concentrations between 1 and 20 mg/L. Diatoms — the brown algae that coats glass and rocks in any new tank — incorporate it directly into their cell walls.
Why it matters
Silicate is the missing piece of "brown algae in new tanks". The standard story (low light + bacterial cycling produces diatoms, which disappear once green algae outcompete them) is mostly true — but tanks with high tap-water silicate stay diatom-prone for months instead of weeks. The diatoms aren't the underlying problem; the silicate they're feeding on is.
How to test
JBL Silicat, Sera SiO₂. Test tap water at source as well as tank water — the source tells you what your starting load is.
What high and low look like
Below 1 mg/L: diatom growth slows to a crawl, tank glass stays clean. 1–5 mg/L: typical tap-water range, diatom bloom likely in new tanks. Above 10 mg/L: persistent diatom problems, even in established tanks; brown coating returns within a week of glass cleaning.
How to fix
If silicate is the root cause and you've ruled out the usual new-tank suspects: use an RO membrane or a silicate-specific resin (Sera Silikatex, JBL SilicatEx) in the filter. RO removes silicate effectively (typical retention ≥95%). Resin works in line but loads up faster than carbon — replace monthly under high-silicate tap water. As a stopgap, more frequent water changes simply move tank water back toward the (still-silicate-laden) tap baseline; the only durable fix is at the source.
