You definately need to return those puffers, they need a recommended 30gal of water per fish, gsp are not a good community fish, need high brackish, and do well in marine when adult. Now I said you need to return the puffers, you could return 2 and get rid of all the other fish and hope for the best with the 2 gsp's.
for them needs to be about 1.015, how ever you cant just jump to this, you should only raise the salinity by 0.002 per week.
Puffers also need a crunchy diet, you should be feeding meaty foods, shrimp preferably raw with the shell, aquatic snails (not mts as shell is too hard), and other such foods.
I wish you all the luck in the world with the gsp, Id love them but I know I cant give them the requirements they need right now