as long as its between full n empty level its fine. can add bit of water once in a while coz its normal for the level in d coolant reservoir to drop a bit.

but dun add too much tho coz once the engine is on n heats up, it might over flow out of the reservoir. there is a tube coming out of the reservoir just for that purpose.

correct me if im wrong but i dont think using just coolant n no water is advisable.