Cyanobacteria (BGA) Treatment: Dosing Your Way Out of the Slime

It’s thick, it’s slimy, it smells like a damp basement, and it can cover your entire substrate in a single weekend. Most hobbyists call it Blue-Green Algae (BGA), but here’s the kicker: it’s not an algae at all. It’s actually Cyanobacteria, a colony of ancient bacteria that uses photosynthesis like a plant but reproduces with the terrifying speed of an infection.

Because BGA is a bacterium, traditional "algae cures" rarely work. You can turn out the lights, scrub the rocks, and cut back on your fertilizers, and yet the green mat often comes back stronger than ever. To beat Cyanobacteria, you have to stop treating it like a weed and start treating it like a medical condition.

How to Identify the Enemy

Before you reach for the medicine cabinet, make sure you're actually dealing with BGA. Look for these "tell-tale" signs:

• The "Sheet" Look: BGA doesn't grow in strands or tufts. It grows in a smooth, viscous sheet that can be peeled off surfaces like a wet blanket.
• The Color: It’s usually a deep, vibrant blue-green, but can sometimes appear dark red or even black.
• The Smell: BGA has a distinct, earthy, pungent odor. If you pull some out and it smells "swampy," it’s almost certainly Cyano.
• Air Bubbles: Because it photosynthesizes so fast, you’ll often see large oxygen bubbles trapped under the slime.

The Root Cause: Why Did It Move In?

Cyanobacteria doesn't just "happen." It is triggered by specific environmental failures in your tank:

1. Low Nitrates (The irony)

Most algae love high nitrates. BGA is different. It is one of the few organisms that can "fix" nitrogen from the air or organic waste. If your Nitrates drop to zero, your plants stall, but BGA keeps growing. This is why BGA is common in "lean dosed" tanks that aren't being fed enough liquid fertilizer.

2. Poor Water Circulation

Cyanobacteria hates "wind." It thrives in "dead spots" where detritus settles and water becomes stagnant. Look for outbreaks in the front corners or behind large rocks—these are zones where flow usually fails.

3. Excessive Organics

Accumulated mulm, rotting leaves, and dirty substrate provide the "snack" Cyano needs to fuel its growth while it waits for the lights to come on.

The Dosing Solution: Antibiotic Protocol

If the infection is widespread, manual removal isn't enough. You need to dose a specialized treatment. The most effective options are:

• Erythromycin (API E.M. Erythromycin): This is a standard antibiotic that targets Gram-positive bacteria like Cyano. It is extremely effective but requires a strict 4-day dosing schedule.
• UltraLife Blue-Green Slime Stain Remover: This is the "secret weapon" for many pros. It is a proprietary blend that is safe for plants, fish, and beneficial bacteria (the kind in your filter), but it is lethal to BGA.

The Golden Rule of BGA Dosing

Because BGA is a bacterium, you must treat it like a course of human antibiotics. Do not stop early. If the instructions say dose for 4 days, you must dose for 4 days—even if the slime disappears on day 2. If you stop early, the surviving bacteria will develop resistance and come back in a form that is even harder to kill.

Accuracy and Displacement

When dosing Erythromycin or UltraLife, the concentration in the water column is vital. Too low, and the bacteria survives. Too high, and you risk stressing sensitive fish or snails.

In a 20-gallon tank with a 2-inch sand bed and large rocks, your Actual Net Volume might only be 16 gallons. Dosing for 20 gallons means you are over-medicating by 25%. In a stressful situation like a BGA bloom, that's 25% more pressure on your livestock that they don't need.

Prevention: Dosing Ferts to Stop the Slime

Once you’ve cleared the mat, you must prevent it from coming back. The best BGA prevention is paradoxically dosing more fertilizer.

By increasing your Nitrate levels to 10-20ppm, you kickstart your higher plants. Healthy, growing plants absorb the resources and block out the BGA. Couple this with a small increase in water flow (add a small powerhead to that "dead spot"), and BGA will never darken your substrate again.

Summary: Take Control

Don't let the green slime win. Identify it, remove it manually, dose a targeted bacterium cure based on your precise water volume, and then fix the underlying nitrate and flow issues. Cyano is a bully, but it’s a bully that can’t survive a well-balanced, well-dosed aquarium.

Written by Richard James

Aquarist, author, and creator of ShrimpKeeper.co.uk. Helping hobbyists achieve professional results through precision dosing.

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