Racers and coachwhips are a bit on the active side, but they will spend most of their time under their hide bark if well fed. I find that mine do OK in a cage about 20 by 20 by 12 high during quarantine, and I expect a 15 gallon aquarium is the minimum for a medium snake like a Black Racer.
I worry about quality of life for my snakes--the standard assumption is that they don't really care about their lives, but humans have a way of assuming things and calling them facts. I try to provide good-sized enclosures for active species, so I like to have mine in a 40 by 20 deep by 12 high at least, following quarantine.
Cypress mulch for a substrate, or Aspen, clean water always available, a bit of cork bark for a hide, and some toys like logs, branches, stones, that you relocate from time to time to mix things up a bit. Racers and coachwhips are slightly prone to skin problens like blister disease, so I like to keep the substrate pretty dry. Occassional moistening is desireable, but the steady humidity appreciated by Corn Snkaes and others is probably too much.
Snake mites apparently think racers are pretty yummy, so treatment of the hide bark or box with Provent-A-Mite, and/or use of pest strips might be something to consider. No sense exposing a herp to pesticides when there are no pests, so use these treatments during quarantine of new specimens, and after that, only when pests are present, but keep a sharp eye on the racers for mites as long as you have them.
Racers and coachwhips feed readily on carrion in the wild, and typically accept thawed mice without hesitation. They never really cuddle like corn Snakes and Boas, but they can be handled easily enough with patience and gentle acclimating. They are beautiful animals, with their satin finish and bright eyes, and I very much enjoyed the time I spent with mine. In fact, currently, I'm getting back into coachwhips and plan to keep and breed them as a permanent part of my hobby.