Hi there,
Figure 8 puffers are a great and rewarding fish to keep. It's a bit like having an aquatic puppy
But, first there's some things you need to know:
1. You need marine salt, not aquarium salt, to maintain brackish water conditions.
2. You'll need a refractometer (ideally) or a hydrometer to measure the salinity of the water. F8 puffers do best in 1.005 max. specific gravity (S.G.). Every time you do a water change you'll need to make sure the salinity matches. If your puffer comes from a freshwater tank then you'll need to increase salinity gradually over a period of time otherwise you'll upset him!
3. There are few brackish water tolerant plants unfortunately. I've tried even with brackish tolerant species and they always end up rotting. I prefer plastic/silk plants, or just go with a totally different decor (I used to have plastic plants but now redesigned my tank so it's more marine looking, with rocks and corals and so on).
Your puffer is an unforgiving fish too, unfortunately. They are very sensitive to water conditions and have a very fussy diet. You'll need to make sure you can meet all of his needs before you bring him in to your tank.
Diet:
Bloodworms, and a regular supply of snails (common pond or ramshorns will do). You MUST feed hard-shelled food, to help file down the beak which puffers have. This beak (i.e. teeth, it's a dental plate) grows fairly quickly, but slower than the South American puffer at least. If you don't feed him snails or other molluscs (like, mussels, or cockles, clams etc) which you can buy from a store (make sure they're not coated in dressings or anything!), then you'll need to perform some dentistry and this is one of the most gruesome and stressful jobs in keeping fish that I've ever seen.
How much you feed your puffer depends on how big he is. The smaller, the more regular (once a day). The larger, the less frequent (once every other day). Puffers will not take flake food, and they should not be fed more than once a day. A fat puffer is an unhealthy puffer!
Conditions:
You will need a tank of 15G
minimum. Puffers are very active and very intelligent, and get bored easily! The larger the space, the more decor you can put in, and the more your puffer will be able to explore. I would go with a 20G for one F8 puffer.
Tankmates:
Not a great idea. I keep two black mollies with mine, and am looking at some orange chromide cichlids -> all of these are brackish tolerant species. Some people have kept bumblebee gobies (BBGs) in with their F8 puffers, but tbh I would not recommend that. They'd make a nice snack one day, I'm sure. Puffers are aggressive fish, and more than that: they are random about the whole thing. One day your puffer will be best of friends with tankmates, and the next day there may be missing fins, or even whole fish!