Installation of separate water tank in builder floor

We stay in builder flat total 6 flat 3 front 3 back, we are on top floor. we 6 flats have 4 water tanks on roof. but being on top floor we were always facing water pressure problems. thus installed separate tank on our roof which is being filled simultaneously with other tanks. we actually disconnected our pipeline from common tank and connected to the new separate tank on our roof. the other 4 out of 5 flats are pressing us to pay more than half of all expenses like electricity bill plus any repair job in motor or submersible pump. In fact out of expenses of Rs 12000 the rest four are paying only 5600 and put balance on our head. which we are not agreeing to because our point is that use of water is similar as when we are using from common tank. installing of separate tank was forced decision as we hardly got water when taps were being used in ground floor or first floor. now my question is that what is right or what is wrong? do we really require to pool extra for electricity plus all repair work to be carried out in the electric motor and submersible pump?? Also pls advise name of some lawyer who can help us in this matter. we are not very well to do family but again being pushed by neighbour to go legal. Many Thanks.