These top screens are used with Nutriculture flood and drain systems, to cover the inlet and outlet holes in the tray to prevent any growing media from entering the pipes and blocking them. They will fit directly into both the 13mm and 19mm inlet and outlet fittings or they can be raised for the overflow with a 35mm spacer placed in first.
Top screens are also used with Aero and DWC Potz Systems, fitted in the lids of the grow pots to allow the heat and stale air to escape easily from inside the pot.