sand filter

[Pooly]

The filter has probably not lost much sand in a year. You say the filter is about 2/3 full. Do you remember the level it was at initially?

Sand filters are designed to use 2/3 - 3/4 of their volume in sand. You need a good gap between the sand and the distributor dish to prevent sand being backwashed away too much.

You could always add a few more inches of sand. If you add too much you will see it in the backwash water.

If the filter is doing its job well, I personally wouldn't worry about it after only a year.