Light interacts with organic material (microscopic-level) in water and creates an environment that encourages algae growth. Algae will rob your plants and root system of vital nutrients that could eventually lead to root rot in some plants. It also makes for more to clean in your system when the growing period is over.
In order to avoid algae in your hydroponic system, you want to block sunlight UV rays (or indoor light waves) from entering the inside of your growing system. If you're a Tower Garden or Farm Stand home grower, these Black Foam Cloning Collar 2-Inch inserts work great at blocking light for empty ports until you're ready to plant into them and thereby reducing algae growth. They can also be used to propagate certain plants like holding securely basil cuttings. Foam cloning collar inserts can be cleaned and re-used up to a point.
Pack of 30