Green Spotted Puffers are actually a brackish puffer when they mature.
As babies they live in fresh water (AKA fresh water puffer)
It would have to be some sort of system that can imitate a fresh water stream merging with the ocean to be able to breed them (I am supposing - not 100% sure!).
hm that seems probable, but expensive... imagine creating some kind of super tank with half fresh water half brackish! @_@
since the spotted puffers come into fresh water to breed i wonder if they would do it while still being kept in fresh water...