Above: slow growing plants (Java Fern in this case) in the path of strong flow (in this instance, the outflow of the lily pipe) are often vulnerable to black-beard algae (BBA).

Stronger Flow is often better

The majority of tanks suffer from insufficient flow, as most hang-on-back filters have insufficient strength. We recommend a filter hourly flow rating of 6X to 10X your tank’s volume.

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 flow can cause mechanical stress to plants that are not used to growing in turbulent water.

Many plants are native to open ponds instead of fast-moving streams. When subjected to strong flow, their leaves, which are not designed to take the constant movement, become susceptible to algae.

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

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 mist, combined with strong flow, is a great combination for plant health.

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

In this picture, notice how the outflow is directed towards open water.