If you do replace all the sand, you should take the opportunity to inspect the stand pipe and laterals for damage. Be careful, the laterals can be fairly fragile. If either of those are damaged, sand would get into your pool little by little and it would explain why the sand was low.
The upside is after doing that, you'll know your sand filter is in great shape. You'll be able to put your mind at ease on that an move on the water chemistry.
The upside is after doing that, you'll know your sand filter is in great shape. You'll be able to put your mind at ease on that an move on the water chemistry.