If the water in your aquarium has gotten cloudy or has
a green tinge to it you most likely have an abundance of
algae growth inside. Generally this will not negatively
harm your fish, however it is very unpleasant to the human
eye. Over time if the algae growth continues it can become
harmful to the fish.
Replacing some of the water over the course of a few days
will help alleviate this problem. Approximately 25% of the
water should be changed daily until the water clears up.
Furthermore, during this time you should try to put a bit
less food in the tank as any excess food will only promote
algae growth. Lastly, try leaving the tank light off as
much as possible. Together this should be enough to clear
your tank up over the course of a few days.
If all else fails there are some additives that you can
buy from your local pet store that usually clean the algae
up fairly quickly. It is important however that you determine
the root cause of your algae growth before it starts again.
If you keep the aquarium in the line of direct sunlight
that is most likely the cause for this algae growth.