No - do not mix straight in the tank!!
I fill up my 5 gallons buckets periodically throughout the week and 2 days, sometimes more, prior to doing a water change, I put the RO water in a 35g trash can (specifically for my water changes), add salt, trace elements, calcium, etc.. and place a power head and heater in the water.
I let that run for a BARE MINIMUM of 24 hours while checking levels during the time its running.
I prefer to let that water go for 2 days...
Once it's done, drag it to the big tank and take a hose and attach to the powerhead and run the water into the tank that way.