My figure 8 definitely makes use of the entire tank. However, when I'm watching the tank he stays towards the front because he thinks it's time to eat. This led
me to believe that he wasn't exploring his surroundings at first.
I feed mine 1 time a day. I've tried a variety of frozen foods, but the only thing it will eat on a consistent basis are frozen bloodworms. I feed it enough bloodworms to round out its stomach. It's very noticeable. They are easy to overfeed as they will try to eat everything you give them.
I don't have any plants in the brackish water so I can't really say which ones might do well. I do understand there is a bit of difficulty involved in having plants in brackish water.
I picked up one of these to help measure the salinity of the water.
I would not recommend getting a hydrometer. You might get lucky and find one that works, but for the most part they are cheap and inaccurate. The refractometer I got from amazon works very well.