Each tank should have had its ...

Each tank should have had its own ball valve to isolate it from the rest of your tanks. The easiest thing to do is drain one tank at a time, remove it to the outside if possible and flush. If the tank is to large to move outside, use a Wet Vac with a PVC extension and vacuum the sludge out after draining. I am preparing to do this very exact thing for a Customer. The accumulated grit from his first tank is eating up his delivery pump every 18 months.