always pre-mix your salt before putting it into the tank, the fish can adjust to changes in salinity(although major changes are stressful) the biggest risk is killing your bio-filter or having a big shift in PH. i mix my salt in a 5g bucket with a powerhead, i just fill up the bucket, treat with some prime, add in the salt(currently i'm at a SG of 1.004, or approx. two tablespoons of IO per gallon) and go to sleep, when i wake up the next day and feel like it i just do the water change, try adding the salt with some sort of measuring instrument so you know approx how much your adding each time, you don't need to be exact but you don't want to really stress out the fish.