The ECR35 is used to add sand, small stones, or even concrete in our PVC feet base for additional weight. Plus Check out our E-Z drain out option which comes with a small hole at the top of the end cap. Caps size fits snug externally a 3.5" x 3.5" O.D. PVC profile. For a permanent use you can add our PVC glue option which will seals the cap to our PVC Feet permanently. This item comes in a Pack of 64 caps, keep in mind you will need 8 caps per pair of wing standards and Schooling standards.