Water wont help at all IMO. What sort of filter do you have in your main tank? If its got two or three sponges in you could use one of these in the new filter [EDIT: if it has a single sponge, you could cut a small section off and use that]. Also, you could add some of the gravel from the first tank as this contains the beneficial bacteria too, not as quick as my first suggestion, but it will speed up the process.
We have bought a product called "Cycle" I think its by nutrifin . Not many people talk about it but I swear by it we have a 55 g tank used Cycle and added amost a dozen fish one being the size of a saucer and they all survived .
People talk about 'cycle' all the time (do a search on this forum), its dead bacteria in a bottle! Think about it, beneficial bacteria need ammonia, nitrite and oxygen to survive, how would they get this in a bottle? I am not saying it is useless, just not good enough to cycle a tank, which needs time to colonise your filter with living bacteria.
I would use some of the sponges from your existing filter to colonise the new 1 along with of the aged water from your tank and maybe some of your gravel. You could mix it with new gravel to give it a good kick unless you wanted to go for sand??? Just let it run for a few days and then stock slowly with say 4 small fish like tetras or maybe 2 platies and then wait 2 weeks and keep testing water for ammonia and nitrite and most of all best of luck JUST DON'T OVER LOAD YOUR FILTER
if you are going to try gravel from the old tank, you could always put it in a knee-high stocking/pantyhose so that is doesn't mix with the gravel in the new tank, but still provides some of the beneficial bacteria!