Quote Originally Posted by cdysthe View Post
The sand filter (zeo sand) had gotten clogged up twice to the extent I had to backwash. When I backwashed what came out looked like skim milk, not the normal brownish looking stuff I normally see.
Are you adding the bleach via the skimmer? If so, my guess is you may be precipitating calcium carbonate, and then trapping it on the filter. This is a GOOD thing.

Another way to check is to catch a bucket full of the backwash, and let it settle overnight. Pour off most of the water. Take a tablespoon of the white goop, and put it in a glass measuring cup. Add 1/2 cup of vinegar. If it bubbles -- it's at least partly calcium carbonate.