Reef Builders says every new tank has an ugly phase, however good the rock or set-up. Diatoms come first and usually fade in two to four weeks. Cyanobacteria and algae often follow, and dinoflagellates less often. Keep up water changes, remove what you can by hand, improve flow in dead spots and don’t rush to change everything at once.
The usual order
Reef Builders sets out the usual sequence.
| What | Looks like | Usually |
|---|---|---|
| Diatoms | Brown dust on sand and glass | First; fades in 2–4 weeks |
| Cyanobacteria | Red or purple slime in low-flow areas | Linked to detritus and dead spots |
| Hair algae | Green strands on rock | Common as the tank matures |
| Dinoflagellates | Brown snotty strands with bubbles | Less common, more harmful |
Don’t panic
Reef Builders’ advice is not to panic or rush, because that usually makes things worse. Every nuisance in this phase is using spare nutrients that the tank hasn’t learned to process yet. As bacteria, coralline algae and grazers establish, they’re outcompeted.
What helps
Steady, simple habits.
- Siphon detritus at every water change
- Improve flow in dead spots
- Wipe the glass and remove algae by hand
- Add a small clean-up crew once the tank has cycled
- Use RODI water; Reef Builders says lingering diatoms can point to silicon in the make-up water
- Act quickly on dinoflagellates; Reef Builders says remove them as soon as they appear
When it isn’t just the ugly phase
If one nuisance lasts more than a couple of months, or corals start to suffer, treat it as a problem rather than a phase. Each linked guide covers what to do.
Common questions
How long does the ugly phase last?
Diatoms typically fade in 2–4 weeks. The whole phase often runs through the first few months.
Should I add more snails?
A few grazers help. Don’t overstock: once the algae is gone, they need food.
Should I turn the lights off?
Reef Builders describes short blackouts for cyano and dinos specifically, not for the whole ugly phase.
Sources and limits
Last checked 13 October 2026. This guide brings together the published advice below. Product instructions come first, and an experienced local shop or club can help with anything specific to your tank.
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