One method is to use plastic sheet covers. This is an easy solution if you are having an algae problem. The coco coir rolls work great in trays. The coir mats are reusable if you clean them. If you are worried about the algae right now, relax, it does not usually create a huge problem. If you do want to clean it use a tablespoon of 35% hydrogen peroxide in a gallon of water. Fill a spray bottle and spray the tray (its fine if the plants are in it).