Algae
Everything is coated in brown dust
A brown, dusty coating that wipes off easily and appears in a tank under three months old is a diatom bloom. It is a phase, not a fault: diatoms take advantage of the surplus nutrients in a tank that has not yet found its balance, and they fade as the tank matures. Wipe it off, keep to your water change schedule, and resist buying anything.
Watch it
Test these first
Before anything goes into the tank and before anything comes out of it, get these numbers. They cost fifteen minutes and they decide everything that follows.
- Nitrate — the nitrate ion — note that drinking water standards are written as nitrate-nitrogen, which is a different number
- Ammonia — total ammonia — the ammonium ion plus free ammonia, added together
What is usually behind it
| Cause | How to confirm | Time to resolve |
|---|---|---|
| A new tank passing through its diatom phase | The tank is between three weeks and three months old, the brown film covers glass, decor and substrate evenly, and it comes off with a light wipe rather than needing to be scraped. | Two to eight weeks, resolving on its own |
| Very low light in an otherwise nutrient-rich tank | The tank light is dim or on for only a few hours, plants are struggling, and the brown film is worst in the darkest corners. Diatoms tolerate low light better than most green algae. | Three to four weeks as plant growth picks up |
| New substrate or hardscape releasing silicate and nutrients | The bloom started shortly after new sand, gravel or stone went in. Fresh material often carries dust and soluble content that feeds an early bloom. | Four to six weeks as the new material stops releasing |
| A maintenance interval that has slipped in an older tank | The tank is over six months old, which makes a genuine diatom phase unlikely, and nitrate reads above 30 ppm. The brown coating is patchy and mixed with other growth. | Two to three weeks of restored schedule |
A new tank passing through its diatom phase
How to confirmThe tank is between three weeks and three months old, the brown film covers glass, decor and substrate evenly, and it comes off with a light wipe rather than needing to be scraped.
What to doWipe the glass at each water change, siphon what has settled on the substrate, and otherwise carry on normally. Nothing you add shortens this reliably; the tank finishes it on its own once biological competition establishes.
Time to resolve: Two to eight weeks, resolving on its own
Very low light in an otherwise nutrient-rich tank
How to confirmThe tank light is dim or on for only a few hours, plants are struggling, and the brown film is worst in the darkest corners. Diatoms tolerate low light better than most green algae.
What to doRaise the photoperiod toward six or seven hours rather than lengthening it dramatically, and let faster-growing plants take up the nutrient surplus. Do not swing to twelve hours — that trades a brown problem for a green one.
Time to resolve: Three to four weeks as plant growth picks up
New substrate or hardscape releasing silicate and nutrients
How to confirmThe bloom started shortly after new sand, gravel or stone went in. Fresh material often carries dust and soluble content that feeds an early bloom.
What to doVacuum the surface layer at each water change to take the settled material out rather than stirring it back in. Rinse any further additions thoroughly in a bucket before they go in the tank.
Time to resolve: Four to six weeks as the new material stops releasing
A maintenance interval that has slipped in an older tank
How to confirmThe tank is over six months old, which makes a genuine diatom phase unlikely, and nitrate reads above 30 ppm. The brown coating is patchy and mixed with other growth.
What to doReturn to a weekly change with substrate vacuuming. In an established tank a brown film is usually a nutrient story rather than a maturity story.
Time to resolve: Two to three weeks of restored schedule
What not to do
These are the moves that make this particular problem worse, and they are all common. Most of them come from wanting to do something substantial when the right answer is something small.
- Do not tear down and restart the tank. This is the single most common overreaction to a diatom phase and it guarantees another one.
- Do not buy an algae treatment for a bloom that ends by itself in a few weeks.
- Do not scrub the substrate violently. Siphoning the surface removes the material; stirring the bed just redistributes it.
- Do not add livestock specifically to eat it. An animal bought as a cleaning tool is still an animal with its own requirements.
Keeping it from coming back
A diatom phase in a new tank is closer to weather than to a mistake, so the useful preparation is simply not to panic and not to spend money. Rinse new substrate and stone properly before they go in, start the light at six hours on a timer rather than leaving it on all evening, and keep the water change schedule from week one. Tanks that are set up patiently still get diatoms; they just get them for less time.
Reference for this page
Equipment involved
- Test Kitsknowing the number instead of guessing it
- Decorative Stones & Pebbleslooks, with the caveats stated plainly
- Filter Media & Cartridgeswhat actually does the filtering, and when it gets replaced
Related problems
Watch it
Act this week
Act this week
Published sources
Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.