The best method is prevention. If you keep the solution away from light, i.e. keep the lids closed and all openings sealed, you can prevent algae from growing. If you already have algae in the system you can remove it with a brush, or use hydrogen peroxide (3ml of 3% H2O2 per gallon of solution) to remove it. If there are particles floating in the nutrient solution, be sure to flush the reservoir and growing chamber with ample water, and then start with a new batch of nutrient. If the problem is severe, make sure you have the algae completely flushed out to avoid the risk of clogging some of the flow lines in the system.