transferring 55 to a 90 gallon.. question about water.

[halfpint720]

I just purchased a 90 gallon reef ready tank and plan on transferring my 55 gallon to the big tank. Just one problem.. i don't have enough water
I was wondering if i can do a water change with the 55 and keep the old water and then when I fill the 90 tank use the old water plus add some fresh with some store bought salt water? Would that work? How long can i keep water in buckets and before it goes bad? lol is that a stupid question?
Plus, is it easy do build your own megaflow overflow kit or should i just buy one for 70 bucks?

 

[flower]

Welcome to the site.
The water has very little good stuff on it...Live sand and live rock, which you will need more of, makes this a new system unless you have already cured live rock.
So my advice is to make new water up set up the new tank and if you can ...wait for a cycle. With a power head to churn the water it won't go "bad" but it will evaporate.
I don't know about the overflow.