1. Sand. These little dudes kinda curl up at night and settle on the bottom to sleep. The first few times you see it you will think they are dead. Fear not! They'll regain their color over 5-10 minutes and start moving around normally... and probably begging for food. Cause that's what puffers do. Since they spend time on the bottom and have no scales, less pointy bits in your substrate = better. Cleaning is actually easier than gravel, you kind of wave the gravel vac a couple inches above the sand and all the poop just floats up and out. Of course it shows waste much more easily than gravel, so you'll probably be using the gravel vac more. Better for your puffer. Don't be lazy.
2. Plants are a huge pain in the butt in brackish water, in my experience. Everything I try dies. Then again, I have a Green Spotted Puffer and he's in almost marine conditions at this point, so plants are good as dead when they touch the water. Hopefully you have better luck in lower brackish (I do not believe figure 8's demand the high salinity of GSPs as they age / grow). Java fern and Java moss are the two most commonly recommended brackish water plants I can think of off the top of my head. Hopefully others will chime in here, as I'm only just getting in to planted aquaria and am only dealing with full freshwater plants. Remember if you do go planted (which again, should be possible in low brackish) you'll want a filter that can be run easily without activated carbon. I recommend an Aquaclear 110, since the carbon is easy to take out and replace with filter floss (buy it as pillow stuffing at fabric stores or Walmart -- much cheaper, exact same thing) to add better water polishing and more area for a biofilter to establish.
3. I'd recommend 50% weekly to start. Please, please use fishless cycling to prepare your tank! Introducing a scaleless fish like a Figure 8 or GSP to an uncycled tank is a recipe for disaster. A 50% weekly change is not a huge deal if it's just a 20G tank. Couple buckets and you're done. Watch water parameters -- there's a good chance you'll eventually be able to go 2+ weeks without them getting out of hand.
4. Snails are really good to wear down teeth. Last thing you want is to end up playing dentist. The rule of thumb for GSPs is snails roughly the size of their eye, so only the tiny ones. I recommend setting up a snail breeding tank (or you can probably just bring them from work). It's easy. Snails are pretty chill like that. I also feed live ghost shrimp purchased at my LFS. Petsmart and such tend to overcharge like crazy for ghost shrimp ($.33 each? Seriously?) so hopefully you can get a better price than that. Some puffers will ignore shrimp, though. One note here: Don't overfeed krill / mysis / other shrimp. They have a compound in them that can build up over time and basically give your puffer lock-jaw. Varying their diet is important. I generally feed krill, blood worms, tubifex, some live ghost shrimp, Hikari carnisticks (he hates these and I pretty much have to starve him before he'll look at them, but really, variety is important).
5. Figure 8s are pretty mellow and stay smaller than GSPs. In hindsight, I probably should've gone with a figure 8 over a GSP due to space constraints and a desire to keep other fish in the tank (my GSP is a murderer, flat out -- everything I've tried has ended up with fins missing within hours, returned for its own good). Hindsight is 20/20. Figure 8s do best in low to medium brackish water. They are not as long lived as GSPs, which reach 15 years regularly, with a lifespan of around 5 years. They also don't reach the size of GSPs, which can hit 6-8" eventually. Most Figure 8s rarely exceed 3".
A 20 long should be plenty for a single figure 8. If you go up to something like a 30 breeder two could be kept together. They can be territorial and are very inquisitive and active, so have a lot of decorations to break up line of sight from one area of the tank to another to reduce aggression and give the fish something to explore. If it starts "pacing" up and down the glass it is more or less "bored" -- it has investigated everything in the tank, found no points of interest, and is looking for a new area to explore. Of course, being in a glass box this search isn't likely to come up with much. Over-filter heavily! Puffers are messy eaters and can be lazy / let things rot. I use a Penguin 350B on a 20 gallon long for my single juvenile Green Spotted, if that gives you any idea. Again, and Aquaclear 110 is probably a good choice. I plan to replace my Penguin (due to noise) with a huge canister rated for 150G tanks.
These little guys will get to know who feeds them and watch you very closely more than likely. I do not believe there is any sexual dimorphism (no way to determine gender) in Figure 8s, much like GSPs.
Any other questions feel free to ask, I've been keeping and reading about brackish water puffers for a couple years now.
