questions about figure 8 puffer

[GoldenPotto]

i recently got a figure 8 puffer and i have a few questions (first time posting here so let me know if theres anything i need to add)

my first question is about feeding, whenever i feed my puffer he will seemingly inflate way more than i fed him, get full and stop eating, then about an hour later he will deflate and still be hungry, is he just sucking in water or is something wrong?

my second question is about feeding snails, i currently feed him a diet of ramshorn snails, frozen shell-on prawns and mussel (sometimes bloodworms as a treat) but for some reason my little guy just sucks the head of the snail out the shell and loses interest so he doesnt chip down his beak, was wondering if theres anything i can do to entice him to bite into the shell

also incase you need them

nitrite= 0
nitrate= <20 ppm
ammonia= 0
ph= 7.5

(not currently brackish as i have some java fern in the tank and im hoping if i acclimate over to brackish over a couple months they will survive (also my water is quite hard)

[Pufferpunk]

You appear to be offering them a good, varied diet. Crush the snails. As they scoop the meat out their beak will graze the shell, helping to keep them worn down. You can also feed mussels on the 1/2 shell.
library/puffers-in-focus/fig8/