Green spotted puffers and figure-8 puffers are of course both brackish water fish, but their salinity requirements are somewhat different. Figure-8 puffers prefer salinity at the lower end of the range, around SG 1.003-1.008, whereas green spotted puffers are better at middling salinities, SG 1.005-1.015. Some folks keep them in marine tanks, but this isn't strictly necessary. In any event, I wouldn't go out of my way to mix them, though juvenile GSPs mix just fine with figure-8s. Once the GSPs get bigger, it's perhaps best to either keep them singly or in groups of their own kind, to minimise any risk of the smaller figure-8s being bullied.
All these puffers have the potential to be territorial, particularly sexually mature males, which in the wild guard nests and spawn. But in practise they usually work well in groups, provided they're given enough space. Allow 10-15 Imperial gallons per figure-8, and at least 20 gallons per green spotted puffers.
If you want a schooling pufferfish, your best bet is the South American puffer, a truly freshwater species. Some Carinotetraodon species are also good in groups, not because they're schooling fish, but because they're so small, it's quite easy to allow them all the space they need.
Cheers, Neale
