Green water appears as just that – your aquarium water becomes bright green and opaque. It’s not at all harmful to your livestock (in fact, they may appreciate it!) but it does look unsightly and can affect plant growth due to its ability to block light.
It commonly appears in tanks that are still establishing, but will also appear when there is too much light coupled with excess organics from waste and overfeeding. Warm temperatures will also encourage green water in combination with these other factors.
If your tank is still in the process of cycling or has only recently been set up, rather than treating, it might simply be a case of waiting for the aquarium system to stabilise.
If the tank is more mature, you may need to reduce your light period, or raise your light so it is not as intense. 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.
Performing a 3-5 day complete blackout can be effective in treating green water, especially when combined with use of a UV steriliser. The base causes should also be treated as green water can quickly return.