Not enough water circulation/agitation?

That’s right – water circulation is also a factor in the growth of unwanted algae!
The flow of water in your aquarium transports nutrients and CO2 throughout your plant mass, and if you have any “dead” spots where water is not flowing, nutrients can settle in those areas instead of being delivered to your plants, allowing algae to use it up instead.
Circulation is also important in moving waste through your tank to your filtration, to be converted into plant-usable compounds.
You can increase circulation either by upgrading to a more powerful filter or by adding a small circulation pump.
On the subject of water circulation, agitation of your water’s surface is also important. If the surface of you aquarium water is too still, a slimy protein film can form, blocking the exchange of oxygen/gas with your water. This gas exchange is required for livestock, bacteria and your plants to remain healthy! It’s simple enough to adjust this by pointing your filter outlet towards the surface of your aquarium. A slight ripple is all that is needed.