i'd suggest you get a hydrometer then. as your puffer, of the green spotted variety, will need to be in about 1/2 strength sea water. or about 1.010sg. if your not sure about specific gravity (sg) just do a google search, theres loads of info on there.
to put that into perspective, your tank with 1 tsp of salt is still gonna be at around 1.000sg (full fresh).
i will say now though, that when you do add saltwater to the tank, your plants and probably your cories wont survive for too long. also GSPs will eventually need to be in full marine (1.022sg +), or atleast high end brackish (1.018sg) as adults. and they get quite big around 6". so you should deffo think about an upgrade. something about 30g minimum. some will recomend a 55gal.
if you want puffers with a cory in FW with plants. there are 2 species i can think of. the 1st are 'red tailed red eye' puffers. they get to around 1.5" and are cool fish. the second are 'pea/indian dwarf/malabar/dwarf (so many names) puffers. they stay small, less than an inch, so would be ideal for your tank. but they can be hard to get feeding. they have tiny mouths, so finding food items small enough that they want to take can be a PITA.