The existing cooling tower chemical treatment company can help you in this regard. Just ask them for details.
Online chemical cleaning can be done with slight dip in pH. One must be prepared for emergencies like cooler tube leaks in this case.
Offline chemical cleaning can also be done which is also called as inhibited acid cleaning. Coolers would be isolated in this case. Coolers would have to cleaned separately. This method would require a jump over between supply and return line at each cooler.