White flakes swirling around attic tank

After 20 years, lifted lid on attic tank. Shock horror. The rim shelf had white sandy deposits, the water had white flakes floating on the surface and swirling around. Bottom had brown sandy deposits. When tank was drained, white stuff turned to brown sandy sludge. This caused by hard water ? No sign of anything falling into tank from it’s cover.

Couldn’t attach the swirling vid.