I'd go for an archer or three, a violet goby and maybe some of the other gobies like jade gobies. As for special equipment, the only thing you need in addition to what you'd have for a FW tank is marine mix and hydrometer. Other than that, just filter, heater, all that fun stuff. Maybe a new garbage can or something for premixing water.
Wow...Those are really cool. But they are standard, I knew what standard monos looked like, I m just trying to get a size comparison. How many dwarf monos can I put in a 125? Would a puffer be a good tank mate with Monos?
Dwarf monos look like M. argenteus, except they have different colors on the tips of their anal and dorsal fins. One's yellow, the other's orange, but I forgot which was which. They grow to only 3" or so.