Stronger is better?

Above: Slow-growing plants (in this case Java fern) in the path of a strong current (in this case the outflow of the lily pipe) are often susceptible to black beard algae (BBA).

Stronger Flow is often better

The majority of tanks suffer from insufficient flow due to the lack of strength of most hanging filters. We recommend a filter hourly flow rate of 6 to 10 times the volume of your tank.

Better flow improves CO2 and nutrient distribution (even if you don’t inject CO2). Increased flow also helps to sweep floating organic matter into the filter, reducing this #1 algae trigger.

So stronger flow is indeed generally better, most of the time.

But strong flow can cause problems

However, strong currents can cause mechanical stress to plants that are not accustomed to growing in turbulent water.

Many plants are native to open ponds rather than fast-moving streams. When exposed to strong currents, their leaves, which are not designed to withstand the constant movement, become susceptible to algae.

Especially Black-Beard / Brush Algae (BBA), which appears as furry black turf.

BBA is frustrating because it can occur in otherwise well-managed, healthy, thriving tanks.

It favours areas of strong flow coupled with CO2 misting.

So hardscape and leaves in the path of the filter outflow are most susceptible.

As with other form of algae, its occurrence can be triggered by weakening plants (diminished health in a mature tank) or a disruption in tank stability (clogged CO2 diffuser, a break in maintenance routine, livestock death etc.) Go here for more details on BBA.

Once it appears it is annoying persistent.

5 things you can do to avoid BBA:

1. Use a lily pipe (see picture below) instead of a jet-style filter outflow. This spreads out the water flow more evenly.
2. Move vulnerable plants (Anubias, Bucephalandra, Alternanthera reineckii, Mosses) away from the direct path of strong flow.
3. Direct the filter outflow towards open water, away from hardscape.
4. Remove old, affected leaves (they will not heal).
5. Especially for mature tanks, provide comprehensive nutrition for ongoing plant health.

Above: CO2 fog combined with strong current is a great combination for plant health.

However, it also creates the perfect conditions for BBA to grow if susceptible plants and hardscape are placed too close to the direct path of the flow.

In this picture, note how the flow is directed toward open water.