Someone said you can use Excel to kill it - this is true. However, overdosing with Excel may also kill off some fish, so be cautious with that.
You will continue to have algae issues until you sort out the cause. Possibilities include:
- direct sunlight
- too much light - put your lights on a timer, not more than 6 -8 hours per day. This will stop future algae. To rid of current algae, you need complete darkness for 4 days which may not be good for the fish and plants in the tank
- over feeding - do you fast your fish one day per week?
- over stocking
- too many nutrients - do you add ferts? Cut back on this. Do you have enough live plants to use up all the nutrients in the tank?
- a too clean tank makes it VERY easy for algae to multiply
While adding an algae eater of some sort will help, usually they can not keep up with algae in a tank which already has a large amount. And remember they do not eat ONLY algae, you still have to feed them!
You may do best by removing the current algae manually, and then changing things which caused it to develop in the first place.