AX2 - Fits Multi-Pore and Undertow "E" and Clear-Free Premium Undergravel Filters (2/pkg.)  $2.99

AX3 - Universal - Fits ALL UG filters (2/pkg.)  $2.99

AX4 - Fits Undertow and Perfect-A-Flow (2/pkg.)  $2.99

AX5 - Use in any Power Filter (2/pkg)  $3.99