If you've noticed a blue-green, carpet-like substance covering everything in your tank, it's likely blue green algae (BGA).
But don't worry, it's actually a bacteria that can photosynthesize. The best way to get rid of it is to deprive it of light by "blackout" your tank for three days. Just cover the entire tank with a sheet and turn off the CO2. Your fish and plants will be fine during this time, but remember to do a water change before and after the blackout.
The most common causes of BGA are low nitrates, dirty filters, poor circulation, and infrequent water changes. To prevent it from coming back, make sure to address these issues and be consistent with your tank maintenance.
