Well thanks a lot for all the replies, the reason for topping off is so that there's not much air in bottle to spoil the cider, I read this is important in, 'The first steps of wine making' can't remember author. Might try the marble idea instead of the cement heard that cement doessn't taste...
I have two gallons of cider currently in second fermentation, but i need to top them up what should i use? Would store bought apple juice be ok or would store bought cider be better or just water?