Cyanobacteria in Aquariums: Identify Slime and Act Safely

Does the growth behave like cyanobacteria?

Colour is not enough. Cyanobacterial mats may be blue-green, dark green, brownish, reddish or nearly black. The University of Florida IFAS marine-aquarium guide describes them as microscopic photosynthetic organisms that can form large coloured mats and notes siphoning during a water change as a removal method. In freshwater planted tanks, use the following field clues together:

• Cohesive sheet: it lifts, folds or peels more like a thin film than separate hairs.
• Fast surface coverage: it spreads across substrate, plant leaves or hardscape as a connected skin.
• Trapped bubbles: bubbles may collect beneath or within the mat under light, but this is not unique proof.
• Distinctive earthy or swamp-like smell: a supporting clue only; do not deliberately inhale material.

Filamentous algae pulls into strands, diatoms usually wipe away as a dusty brown coating, and mulm breaks apart as loose debris. Mixed growth is possible. If the identification remains uncertain, remove a small sample mechanically and obtain a clear close-up for a qualified aquarium professional before choosing a treatment.

Plan one controlled removal session

Before the water change

1. Check livestock behaviour, temperature and filter flow.
2. Prepare replacement water using the normal species-appropriate method.
3. Position a siphon close to the mat before disturbing it.
4. Use dedicated aquarium tools and protect cuts or irritated skin; wash hands after aquarium work.

During removal

1. Lift an edge gently and siphon the loosened sheet at the same time.
2. Remove badly smothered or decaying leaves rather than shredding the mat in place.
3. Work in small sections. Do not stir the entire substrate to chase every fragment.
4. Clean accessible intake guards and obvious debris pockets without dismantling all filtration.

After refill

1. Restart required equipment and confirm normal circulation, temperature and leak-free fittings.
2. Dispose of removed material; do not return tool rinse-water to the aquarium.
3. Photograph the cleared zones so recurrence can be compared at the next observation.

Find the condition that lets the mat return

Do not turn this table into a universal nutrient prescription. Aquarium measurements are meaningful only with the correct test method, water context and livestock needs. Correct the observed mismatch rather than chasing a single internet target.

What about a blackout?

Darkness may reduce photosynthetic growth, but it does not remove trapped waste, restore a blocked flow path or identify why the mat appeared. A blackout is not a substitute for removal and diagnosis. If one is considered, protect livestock oxygenation, follow species-specific guidance and do not combine it with a chemical treatment whose ingredients, dose or compatibility have not been confirmed.

Track recurrence by place, not by impression

For the next few maintenance visits, record the first location where the mat returns, time since removal, light schedule, flow changes, feeding changes and plant deterioration. A smaller patch returning in the same low-flow pocket points to a different next step than new mats appearing throughout the aquarium.

• If it returns only in one pocket, improve access and local flow.
• If it follows repeated decay, correct the plant or husbandry problem creating that material.
• If it spreads rapidly despite careful removal and stable maintenance, seek qualified identification and treatment advice with the livestock list, tank volume, recent changes and clear photographs.