1)you have stated that there are 6 flats in building and 4 water tanks installed . since you were facing water problems you had separate water tank installed .
2)maintenance of common water tanks is the joint responsibility of all flat owners . you cannot shy away from your responsibility from shelling out money for maintenance of common water tanks merely on ground you have installed your own separate water tank .
3)if you have any grievance you can file suit in district court . with consent of parties dispute can be referred to lok adalat .
4) if no settlement is arrived at dispute would be referred back to trail court for disposal .
5) your chances of success are bleak .
6) for lawyers in Ghaziabad you can search in kaanoon website for lawyers practising in said area . MR N addem Qureshi Advocate s from Ghaziabad . you can contact him if you so desire .