How to check sand level in Cristal Flo II sand filter, think builder under filled


Active member
Mar 17, 2014
Albq. NM, United States
Long story short. I have very little confidence the pool builder we used. It seems our Cristal-Flo II 3.15 sf allows a fair amount of either sand or silt through the filter and back in to the pool. I suspect it's silt because when brushed it disperses easily (my readings here indicate that is likely silt and not sand). Also, upon pump start up all returns have a light brown cloud shoot into the pool for 15-20 seconds. My suspicion is that original set up of the filter was not optimal. Either not enough sand, too much sand or damaged in the process. This builder had a couple of real hacks working for him and I happened to walk by and see the worker putting sand into the filter. He was struggling and seemed very frustrated by the process. I would not be at all surprised if he got the absolute bare minimum in and gave up (seemed to be his MO on everything he did). Two questions, would too little sand create a condition of poor filtration and allow more silt through than the correct amount of sand would as well as the brown plumes I see on start up and, (what I really woud like to know) if I were to pull the multi valve and visually inspect, how much or what depth should the sand be (looked online but only found volume references)?

The pool is one year old, Pentair VS pump, sand filter, has automatic cover that is closed most of the time. I can close the cover with the pool clean and when I open it there is always silt on the bottom (sort of lines of silt). Robotic cleaner mostly has silt in basket after cleaning. I'm hoping to inspect and find sand level borderline or low and bring up to spec and cure problem. Any input would be appreciated. Thanks!


TFP Expert
Feb 3, 2014
Central Minnesota
I believe my Hayward filter with a top mount multiport is about 2/3 full of sand, maybe a little higher when I filled it last year. If your sand level were too low, you'd only be able to tell by having to backwash more frequently than if you had more sand as you'd likely have slightly lower filter pressure than normal and it would rise 25% above clean sooner. It would also take longer pump run time to achieve the same water clarity. When you have silt or sand passing through the filter, it's more likely that they used improper sand or damaged the lateral fins during installation.

Pool sand is properly sized #20 sand that is designed to be very uniform in size so that it does not pass through the laterals and provides filtration. The other possibility is that proper sand was used, but the laterals in the bottom of the filter were damaged during installation. I'm afraid your best bet is to open the filter, allow the water to drain and sand to dry somewhat then use a shop vacuum to suck out all the sand, then inspect the laterals. Replace laterals that have cracked or broken fins if you find them then refill with properly sized pool sand. If you're lucky, they did not damage the laterals and were cutting corners by filling with beach or sandbox type sand. Be sure when you refill, you cover the center stand pipe of the filter with a plastic bag and hold it in place with a rubber band as you don't want sand in the center pipe. Also, partially fill the filter, up to half full, with water to help cushion the sand around the laterals when filling and fill slowly.