The main root cause of algae is too much light and too many nutrients. If the water in the aquarium is hit by direct sunlight, you are very likely to get the green algae.
To avoid having algae:
- place the aquarium to avoid direct sunlight;
- do not add too many nutrients to the water (not too much food for the fish);
- install proper filtration; a UV lamp will be very beneficial to keep green algae under control;
To get rid of the algae, you have two choices:
- If you are patient, follow the step above and you might be lucky to get rid of the pest.
- Start everything fresh: wash and heat treat the gravel, wash the devices and the inside walls of the aquarium chemically to really kill everything, throw away the plants (or try to wash them with a solution of H2O2). The cycle the aquarium and enjoy.
Right now, your water might look green if the walls of the aquarium still have algae. To verify, just put some water from the aquarium in a clean transparent white glass and look at the water - is it green or not? If not, then yo need to clean the inside walls of the aquarium. If you have good filtration, use a razor for aquarium to remove those algae.