It doesnt need to be precise.. just use a good quality marine salt, and get yourself a refractometer off fleabay for £20... that is the best way of measuring the salinity of you water...
then just put him across in fresh... and after a week or so of settling bump the tank up to 0.002...
leave the plants out as they will just rot in the rising salt....
use the time whilst you are waiting week by week to research marine and you can (if you want) slowly build the tank up to full marine and get his some marine fish buddies...
that what i did, and many others have done. My GSP is in full marine and has previously lived with clownfish, drawf angels, valentinis... only reason he now doesnt is because i keep reef tanks, so he now has his own tank again.
If going marine (slowly) seems to much of a jump, just fill the tank with loads of interesting rocks and possibly fake plants and keep bumping the salt week by week at 0.002 (its easy, honest) until he is up around 1.018 ish..

