Figure 8s can live in either brackish or fresh, but I've been told that they tend to do better in light brackish (at around 1.005 sg) -- they'll grow a bit larger, live longer, and are less susceptible to disease. At least according to people I've talked to who've kept them for years... I trust their experience.
There is a lot of debate about whether figure 8s are true freshwater fish or not -- a lot of what is written about them is based on the location where they were collected (they are typically wild caught), which can either be a fresh or brackish site. Plus, some say that while they are rumored to live in freshwater in the wild, they do better with brackish water in captivity.
Anyhow, after hearing both sides of the debate, I've decided to keep my puff in brackish. He has been in brackish for 2 years now, has never been sick, and is very healthy and happy. He's very active and smart! Like yours, he'll eat virtually anything. I also soak food in vitamin water... he loves snails, every type of shrimp known to mankind, earth worms, plankton, scallops, fish flakes, crab meat, and lobster. He likes live, fresh, frozen, freeze-dried, pelleted, flaked... you name it...
I think one of the most important things in keeping a puffer healthy is keeping the water as perfect as possible. Figure 8s are hardy for puffers, but they simply won't handle ANY ammonia, nitrite, or even much nitrate. I do partial water changes once a week to keep the water pure. But the two main reasons why I want to plant the tank is because a) I want to keep nitrate levels as close to zero as possible, and b) plastic looks kind of tacky.
Like I said, my puff DOES nip the java fern, but I found that the java moss works well... it isn't leafy, so he doesn't shred it. It also seems to do well with the salt.
Thanks for the links!