what would be best is to remove a quarter of the gravel and replace it with some sand, wait a week and remove a quarter more of the gravel and replace that with sand.. and so on and so forth until the gravel is completely removed.. this way you wont re-cycle your tank.
and a hydrometer is about $5-8 dollars (u.s.)
a refractometer is more acurate, however more expensive about $50 and up
you'd be fine with the hydrometer, just make sure it can read specific gravity of the lower levels (about 1.004+) so you can measure your salt content.
as for snails, they take no effort at all to culture, a fish bowl will work and all you need is 1 pest snail (pond snail whatever you want to call it) and soon enough you will see clear egg sacs all over the glass. problem is, you prob. wont be pumping out snails as fast as your puffer eats them. my dwarf puffers (freshwater puffers) eat about a snail a day and live blackworms. i culture both so its not a problem, but my snail population is looking weak, they just eat them so fast lol and theyre in a 30gal tank seperate from the puffers .. so you'll want to supplement other foods then snails to eat, im not sure about the turtle food that might have nutrients in it unhealthy to the fish but who knows.. if your going to use that as a supplement for snails i wouldnt do it for a period of time..