You need to provide the plants the nutrients they need to grow. You can use fertilizer tablets shoved under the rooted plants, or you can add fertilizer to the water, but the plants will only remain healthy if you do one or the other. Your lighting is pretty low, so fertilizing once a week is probably all that you need. You need to provide nitrates, phosphates, potassium, and trace elements. Read Rex Grigg's website about fertilizing for more information.
Rex's Guide to Fertilizers Once you get healthy growing plants, any algae issues will be greatly reduced.