How to fix green Aquarium water
Green water is the result of excess algae inside the aquarium and the growth of this algae can be very fast.
If your aquarium is in direct sunlight for part of the day this will help to encourage algae growth. If this is the case then you need to stop sunlight from directly hitting the aquarium. A relocation of your aquarium may be another option because direct sunlight will keep giving you problems over the years.
Another reason to green water can be incorrect lighting in the aquarium. Most aquarium lighting especially strip lighting needs to be replaced every year. LED lighting will last longer and can last between 5 and 10 years. Buy a purpose built aquarium light because many household lights are not suitable.
Clean the tank and gravel then replace 25% of the water in the aquarium with clean water. If you are using tap water then treat this with a solution to neutralise the chlorine and other harmful chemicals found in tap water. If you have RO water then use this.
Clean the aquarium filter suing some of the old aquarium water. Never clean the filter media under a tap because the chlorine found in tap water will kill all of the healthy bacteria living inside the filter media.
After a couple of days do another 25% water change but first using a gravel cleaner give the gravel at the bottom of the tank another clean
If you still having problems then you can buy a UV sterilizer and this will clean the algae in the aquarium guaranteed if properly set.