How do you remove iron/hardness scaling/fouling?

What's the preferred way to remove iron/hardness scaling/fouling on trays?

Some people recirculate a mild acid (such as citric acid) to re-dissolve the iron oxides. Mechanical cleaning with a power washer or brush is also common. Some people note that dry scale is easier to remove than wet scale, and use two sets of trays to allow the fouled set to dry prior to cleaning.