GDA appears as a fast growing, green film, usually on hardscape and the sides of aquariums. It’s easy to wipe away but will quickly regrow, sometimes overnight. The difference between GDA and Green Spot Algae (GSA) is that GDA forms a thin green film, with no formation of dots like with GSA.
GDA is often develops in tanks with low plant mass and excess organics, in tanks with high light and nutrient rich water, and is encouraged by warmer water temperatures.
It also frequently appears in tanks that are still cycling or in those with insufficient beneficial bacteria.
First, you should increase your plant mass so that your nutrients and light are being used. Maintain a regular maintenance schedule of siphoning detritus and changing water on a weekly basis to ensure nutrient and organic levels don’t get too high.
If you’re using pressurised CO2, consider if you are injecting the correct amount into your system.
Less severe cases of GDA can be treated easily with products such as LCA Carbon Plus or Triple B.
In severe cases, starving the GDA of all light sources with a 3-5 day complete blackout can be effective. Keeping nitrate levels low during this period until the GDA dissipates may also be helpful.