How much weight a concrete roof of 12x06 feet dimensions can hold. Will it easily support a water tank of 150 gallon capacity ?
Well, basing the calcs on water, that is around 568kg or 1253lbs plus the weight of the tank itself and any associated pipework. Whether or not it will support such a load depends on the thickness of the concrete and whether or not it has any reinforcement.
Adding weight to a basically suspended slab increases the tension on the concrete and generally, concrete is weak in tension (unless reinforced).