It's not abnormal for sediment to settle-out from water that's been sitting 'idle' for a while. Especially if you have well water, you'll want to drain the sediments out if it's been a while since its been done. Municipal water really doesn't have this problem unless there was perhaps a break in the line somewhere and dirt got in the line.
That's one heck of a storage tank! Perhaps others can add to this. My best to ya and hope this helps.