There are some very safe algaecides that can be used to control algae. Fish kills are usually caused by killing too much vegetation at once. This vegetation starts to decompose and uses up oxygen.

It is best to start treating in the spring and prevent the algae from getting bad. It may take 6-12 treatments a season to keep it under control. The algaecide only kill what is there, they can't prevent it from coming back. If the algae gets bad (over 1/3 covered) you should only treat half of the algae and wait a week and treat the other half.

Check out this website for more info www.aquaticcontrol.com.