Silicate (SiO₂)

Silicate is harmless to fish and inverts, but it's the fuel behind the brown diatom film that coats a new tank's glass and hardscape. Keep it under about 5 mg/L and diatoms stay a passing phase; most tap water arrives at 1–20 mg/L, and supplies above roughly 10 mg/L keep tanks diatom-prone for months rather than the usual few weeks. There is no lower limit worth worrying about — you never dose silicate, you only ever try to remove it.

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.

Worth being clear on what silicate is not. It isn't toxic — a tank at 15 mg/L is no danger to a single fish or shrimp, they simply don't interact with it. And it isn't a plant nutrient: unlike nitrate or phosphate, your stem plants don't want any of it, so you can pull it as low as you like without starving anything. That makes silicate the odd one out on this glossary — the only parameter where "as low as possible" is a perfectly good target.

When the brown film is really silicate

Every new tank browns over in the first month; that's diatoms riding the ammonia spike of the cycle, and it clears on its own once the nitrogen cycle settles and green algae move in. Read the first 30 days guide before you blame the water — nine times in ten it's just the tank finding its feet.

The tell that silicate is the real driver is persistence. If the brown coating is still returning within a week of every glass wipe two or three months in, on a tank that has otherwise cycled and settled, the diatoms have a standing food supply — and that supply is almost always the tap. A dusting on new leaves and slow growers like Anubias, wiped away and back by the weekend, is the classic high-silicate signature. Fish and shrimp show nothing, because to them the number is invisible.

Telling diatoms from other growth is mercifully easy: diatom film wipes off with a finger as a dusty brown cloud, black beard algae sits in wiry tufts that won't smear, and green dust comes back green. Diatoms are also the one algae with genuinely enthusiastic natural predators — otocinclus, nerite snails and amano shrimp treat a fresh film as a buffet, which keeps the symptom in check while you deal with the source. Just don't stock an oto squad for the diatoms in a young tank: once the film is gone they need another food supply, and a fish bought as a cleaning tool tends to starve as one.

Does sand or rock feed diatoms?

The question every diatom thread eventually circles: "is my play sand doing this?" Almost certainly not. Quartz sand, silica sand and the aquarium glass itself are crystalline silica, and at aquarium pH they are effectively insoluble — the dissolution rate is slow enough that your tank would take decades to notice. The dissolved silicate feeding the film came in with the water, not the substrate, so tearing out a sand bed to fight diatoms is a weekend of work for nothing.

The ratchet that does deserve attention is top-off water. Evaporation removes pure water and leaves every dissolved solid behind, silicate included — so replacing evaporation with silicate-rich tap adds a fresh load every time while removing none. An open-top tank losing a couple of litres a week, topped up with 15 mg/L tap, climbs steadily even between water changes. Top off with RO or distilled and the ratchet stops turning; the same habit keeps hardness from creeping upward too, for exactly the same reason.

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.

Testing the tap is the step people skip and the one that actually settles the question. Draw the sample after letting the tap run for a minute so you're reading mains water, not what sat overnight in the pipe. If the tap reads 12 mg/L and the tank reads 12 mg/L, no amount of scrubbing or water changing will help — you're topping up with the exact thing feeding the diatoms, and every water change resets the tank straight back to the tap baseline. That single comparison decides whether you're chasing a filtration fix or just waiting out a normal new-tank phase.

One chemistry footnote for when you're comparing vials: silicate kits use the same molybdate reaction family as phosphate kits, which is why the two parameters can weakly cross-read — a heavily phosphate-loaded sample can nudge a silicate result upward. Kits include a masking step and at ordinary hobby levels the effect stays in the noise, but it's worth knowing before you chase a 1 mg/L disagreement between two brands as if it meant something. Testing once or twice a year is plenty for this parameter — silicate moves at the speed of your source water, not your tank — but log the readings when you take them, so "the tap changed" is a fact you can check rather than a hunch.

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.

There is no "too low". Zero silicate simply means no diatom food, which is exactly what you want; nothing in a freshwater tank suffers for the lack of it.

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.

The RO route is the clean one: run your source water through the membrane and it comes out at near-zero silicate, then you remineralise back to the GH and KH your livestock wants. The trap here is thinking a water change alone will help. It won't, if the refill is silicate-laden — a 50% change on a 12 mg/L tank refilled with 12 mg/L tap water lands you right back at 12 mg/L. The water-change impact tool makes that plain: a swap only moves a parameter toward the refill water's number, so when source and tank match, the dilution does nothing. Fix the water going in, and the diatoms lose their supply on their own timeline.