if you are worried about your bacteria colonies, get a cup of live sand from someone else's tank to either reseed your tank or for a mininum to help give you biodiversity.
And as far as bringing down your slainity, do it slowly. 5gals at a time it not slow at all. I would do 1/2 gal or 1 gal at a time at the most. If you want to do more, wait at leat an hour or two before doing another 1/2gal or so. Do you have a means of measuring the salinity yourself?