Yea, the minimum size tank for a redtail black shark would be a 55g, imo. Maybe a 40g breeder.
I must disagree here, My first tank a 20gallon high I had a very happy 5"+ adult RTBS happily in with an angel, swardtail, 5" bristlenose, neons, 1 khuli loach and an albino cory. The tank was purchased 2nd hand with stock (had been set up that way for a more than a few months) and we left it that way for another month at least, no dramas were had with the red tail in ths tank, which it goes to show the minimum tank size stated on more than site for red tails is right at 20gallon, I think they are definetly happier in a 30 though. And I wont go into multi shark tanks here...
Andrew