Setup path, stage 7

The reef tank ugly stage: diatoms, algae and clean-up crew

Brown dust, then green film, then hair algae: almost every new tank goes through it. It is the tank finding a balance, not a sign you did it wrong. The aim is to ride it out, not to fight it with bottles.

Last checked 21 October 2026. How we check

What do you need for this stage?

• Nitrate and phosphate test kits
• A TDS meter for your RO/DI
• A small clean-up crew (snails first)
• A glass cleaning magnet and a siphon

What happens in the ugly stage, and how long does it last?

Keepers commonly report the same order: diatoms (brown dust on sand and glass, fed by silicate from new sand, glass and RO/DI), then green film and hair algae, and sometimes red slime (cyanobacteria). Timings vary widely: diatoms for two to four weeks, the rest for six to twelve weeks, and some say a tank only settles after six to twelve months (typical, check). No one has measured it properly.

Brown, snotty strands with bubbles that vanish after dark and return with the lights are more likely dinoflagellates. BRS links them to nitrate and phosphate near zero, which very clean new tanks often have. Our problem pages cover diatoms, nuisance algae, cyanobacteria and dinoflagellates in more depth.

What should I do about algae in a new tank?

• Check the RO/DI still reads 0 TDS if diatoms go on and on.
• Keep nitrate and phosphate measurable but moderate. Chasing them to zero invites dinoflagellates.
• Run the light at a modest level for about 8 to 10 hours. BRS suggests 10 hours with an hour of ramp each end; most keepers run 8 to 12.
• Remove hair algae by hand and siphon red slime during water changes.
• Fix dead spots in the flow: BRS says cyanobacteria favour them.
• Leave chemical algae and bacteria treatments as a last resort. BRS notes some cyanobacteria treatments can lower oxygen.

When should I add a clean-up crew, and how many?

Once ammonia and nitrite are zero, add a modest crew and let it grow with the algae. Snails are the safest start: trochus, cerith and nassarius snails for sand. Hermit crabs such as blue leg hermits are popular but can kill snails for their shells, so keepers argue about them.

Sources give no consistent number of snails per litre. Start small (a handful per 50 litres), feed the tank as normal and add more only if the algae outpaces them. A crew that starves is worse than none.

What are the steps, in order?

1. Confirm ammonia and nitrite are zero.
2. Set the light schedule (about 8 to 10 hours, ramped).
3. Add a small snail crew; acclimatise slowly.
4. Test nitrate and phosphate weekly and write them down.
5. Remove algae by hand and siphon during water changes.
6. Check RO/DI TDS if diatoms last more than a month.

Your checklist

Tick items as you go. Ticks are saved in this browser only and fill the depth gauge.

What test must pass before moving on?

What are the common mistakes?

• Dosing algae killers in the first months.
• Adding a large crew at once, which then starves.
• Chasing nitrate and phosphate to zero.
• Running the lights 12 hours or more at full power in an empty tank.
• Blaming the tank when the RO/DI membrane or resin is spent.

Where do sources disagree?

Figures from sources read as a search summary only are shown as typical figures: check them before you rely on them. Found a mistake? Tell us how to fix it.