Our most popular tray is the European Leach Tray; it is at the heart of many custom-designed systems. There are two drain holes at one end of the tray that accept either plugs or drain elbows. For most of our custom work we use the elbows. This black leach tray was designed for a 6" wide by 3' long plastic-wrapped rockwool slab; though most any growing medium will work. Each tray comes with 2 elbows.