There are a few factors that can start the growth of algae in your Biosphere:
1) The Nitrate Level is high, Nitrate encourages plant/ algae growth,. Check your plant and make sure that it is still a vibrant green. If the plant has died, it isn't filtering the nitrate as it should.
Another reason is the water in the tank has levels of chlorine. In this case, always remember that when you are doing a water change that you use Distilled Water, do not use regular tap water to fill your
tank when doing a water change, chlorine is very harmful to ADF. You can also use a method of a Dechlorinated solution. It can be bought at most pet stores or aquarium stores.
2) Too much natural sunlight (not an electrical light for the tank), Direct sunlight is algae's best friend, so try to move your tank in a darker area. You may also use a blanket to cover the tank for a week and it will kill algae.
Easy steps to eliminate algae in your tank:
1. Remove enough water to access the algae growing on the side of the tank- try not to remove more than 50% of the water in the BioSphere. Use a turkey baster or similar water extraction tool to suck out any left over food or gunk lingering on the colored gravel. Do not remove the gravel or sand from your BioSphere.
Take a paper towel, napkin, or tissue and wipe the insides of the tank removing the algae. You might need a razor blade if it won't easily wipe off. Remove the 3 large rocks and give them a good scrub under running water. Remove the plant and rinse under running water as well. Replace the plant and 3 large rocks.
2. Cover the tank with a dark blanket or towel to eliminate all light to the tank for a week. This will remove the "food" for the algae.
3. Purchase API Algaefix liquid. Use one drop for our classic size BioSphere and 2 drops for our gallon size BioSphere. Follow usage instructions on the bottle.