Are the brown or red mats dinoflagellates, cyanobacteria or diatoms?

Why this happens

Diatoms are silica-associated microalgae common in new systems, cyanobacteria form bacterial mats, and dinoflagellates include many species with very different toxicity and behaviour. All can appear brown or reddish.

• Diatoms often form dusty coatings that wipe away easily.
• Cyanobacteria commonly forms cohesive slimy sheets.
• Some dinoflagellates make strings and bubbles during the light period.
• Mixed blooms can occur at the same time.

Correct identification matters because aggressive nutrient stripping may worsen some dinoflagellate problems, while a blackout alone may not solve cyanobacterial causes.

What to check first

• Does the material form dust, slime sheets or strings?
• Does it return rapidly after lights-on?
• What are nitrate and phosphate trends?
• Can microscopy or an experienced identifier confirm it?

Compare the result with the aquarium’s normal baseline, not only a generic target. Record the time, recent maintenance, new livestock, feeding changes, source-water details and any equipment fault. A short timeline often separates the original cause from secondary symptoms.

A practical action plan

1. 1. Document before treatmentPhotograph under white light and sample the growth.
2. 2. Remove biomass safelySiphon through fine filtration and avoid aerosol or skin exposure.
3. 3. Stabilise nutrients and biodiversityAvoid forcing nitrate or phosphate to zero.
4. 4. Use targeted controlChoose UV, flow, light or ecological measures only after identification.

How to interpret the response

A slower return and healthier competing organisms indicate progress. A change in appearance may mean the dominant organism changed rather than the problem ended.

Change one important variable at a time where welfare allows, then observe and retest. When several products or adjustments are made together, it becomes difficult to know what helped, what caused stress and whether the underlying problem remains.

What to avoid

Do not taste or smell samples closely, allow pets access, dose broad algaecides, strip nutrients blindly or rely on one social-media photograph for identification.

When it is urgent

Livestock neurological signs, human or pet exposure, widespread coral stress or a toxic-species concern requires urgent specialist advice.

How to prevent it next time

Quarantine wet additions, maintain stable nutrients, use source-water control and keep a microscope or local identification contact for valuable reefs.

Keep enough records to recognise the aquarium’s normal behaviour, water chemistry and equipment performance. Trends are more useful than isolated numbers, and early correction is usually safer than a large emergency change.