stop the algae from growing at all.
 

It would be better to stop the algae from growing at all. Where is the algae growing ?

Please! Algae is you friend. Without algae you have a problem with hydroponics. Have you ever seen a healthy stream in the wilderness without algae. Algae means that their is nutrients in the water that your plants need. It only becomes a problem when it gets so bad that it covers the top of your medium and prevents oxygen from being suck into the medium when the water drains from the cups.