My pool is turning green quite easily lately despite chlorine and ph being in range, so I have to keep shocking it to clean it up. My pool guy says that I need to add sand because my sand level is low and so the filters are not filtering well. I asked him how the sand level got low and he told me that the it gets worn down over time and dissolves. Pool was built about 12 years ago. From what I am reading here, sand does not degrade or dissolve over time as my guy is telling me it does. I haven't been able to find a clear answer on this so I'm asking here. Thanks.