Setting up a 20g for a F8 Puffer

[iceicebaby93]

I have a 20g freshwater tank that currently houses a number of fish that will soon be moving to a 36g freshwater bowfront. I have a juvenile figure 8 puffer that I am going to be moving to the 20g tank. Currently he is in my 15g freshwater tank as that's what he was in when I got him as well, but I know that F8's ultimately need brackish water, but I'm not sure what to do to get the 20g set up/changed to be ready for the puffer or if I need to change anything currently other than the water. How do i know the correct amount of salt to mix with the water so that it's the correct salinity? I have a HOB filter, that should still work right? I know F8's are notoriously messy but it will just be him in the 20g. I'm just feeling a little confused and want to make sure I do everything right for him!