Increase water changes.
Clean substrate during every water change.
Clean out filter media every or every other water change in old tank water.
Clean glass / plants / decor whenever you can.
Reduce lighting (both intensity of the light, and duration of lighting through the day).
If planted, ensure you are fertilizing properly, ensuring CO2
is abundant (if applicable), consider using Flourish Excel / Met14 daily to help keep algae at bay.
Consider amano shrimp / snails / oto cats / other algae eating fish.
Keep tank away from direct sunlight.