Because you removed the chlorine from the water. (Why do you put chlorine INTO a swimming pool? So algae WON'T grow in your pool, right?) Algae isn't harmful. In fact, when the tank light is on, algae puts oxygen into the water. It just looks ugly, plus, when the light is off, it drains oxygen from the water. Leave the light off too long, and algae turns brown and makes the tank toxic for the fish. Solution: consider adding a Chinese Algae Eater to your tank. The name of this fish says it all. It is totally non-agressive, so it can live with other fish. Plus, after it's done eating, you get a sparkling tank! For massive algae infestations, use an algae scrubber. Avoid using chemicals to remove algae as these can harm certain species of fish.

Algae grows in an aquarium because of excess nutrient. Several types of algae can occur under different conditions, but it's all caused by nutrients. Algae is a plant, a very remarkable plant that can grow at extremes of temperature and varried lighting. Brown Algae will tend to grow in tanks with less light, Green Slime and Beard Algae will grow in the presence of more light and Red algae, really a cyanobacteria grows sometimes that can be difficult to remove. The best way to controll algea, and I say controll, because it's going to be there, even if you have trouble finding it, is by paying careful attention to what you put in the tank. Fish food is High is Phosphates, yep that's the stuff you can't have in dish detergent because it causes algael blooms in drinking water. Limiting the amount and types of food as well as water changes to keep nitrates in check is the best approach, avoid sunlight hitting the tank. If you keep plants, make sure they are densely planted and provided with enough iron, this will enable them to out compete the algae for nutrients. Co2 injection is not difficult either, and it makes a more stable enviorment. Fish that eat different types of algae: Plecos will eat just about all types of algae, and plants too. Some get too large, but the bristlenose and other Ancistris sp. only get a few inches long. Other considerations would be Peckoltia sp. and there are some nice ones, but they can be expensive, they all eat plants though. I use them to keep brown algae under control in low light aquariums. Guppies are good for eating green fibrous or hair algae and don't really bother plants. The best for Green hair algea is by far the Siamese Algae eater, or Crossocheilus siamensis, difficult to find and often confused with the Flying Fox, it's the algae eater of choice. Won't touch the brown algae though. Red algae, being a cyanobacteria is somewhat different. I've heard that using antibiotics can rid an outbreak of this, but I've never had an outbreak. Usually algae outbreaks occur when a tank is stabilizing and often will correct themselves, or atleast become bearable with time and normal water changes. Some fish NOT to consider for controlling algae: Chinese Algae Eaters can get really big, sometimes up to 11", and they can be VERY aggressive as the get older. Some only reach about 6", but still, three or four 6" aggressive fish in a peaceful community aquarium will wreak havoc in no time. The Flying Fox is also agressive when it gets older and it's appetite for algae decreases with age. Oto's are good, but so many of the so called "Killer Oto's" have gotten into the trade that it's a good fish to avoid unless you are very careful. So for the unplanted aquarium, the smaller plecos work great, and one is more than enough. For Planted, use the Siamensis, great fish and fun to watch. They actually rest on broad leaf plants, they're so cool. Make sure you are buying a Siamensis!!!!! There are ways to check! Control what goes in the tank, fish food and chemicles, NEVER EVER use chemicles that alter PH, they often contain phosphates and you can controll ph with much more natural means like peat moss or baking soda Never use an algae destroying chemicle either. Light + Nutrient = Plants No plants=Algae because there is nothing to use the excess nutrient. A UV sterilizer or Diatom filter will remove some of the nutrient, but you will always have some algae, it's actually means your tank is healthy. Hope this helps.