Turn up your temp to 78 and more will dissolve.
Also, get a refractometer. you can pick one up from drsfostersmith.com for around $40
hydrometers are notoriously inaccurate.
Give it some time to mix. In a couple days if it's still too salty, take out 4.5 gal
and keep it in a bucket for a future water change. you can add some RO
to the bucket and the tank to get the salinity right in each.