Lyhhrt Trilogy Series by Phyllis Gotlieb

A mature alien woman judge sees an amphibious human woman, obviously a slave, displayed in a tank in front of a sex palace. And so an interstellar plot of murderous proportions involving many races and planets, galactic corporations, exploitive sex and horrible slavery is revealed.

An interstellar alien corporation run by aliens was thwarted in its plans to exploit genetically altered slaves. Now, in an attempt to keep it's case from ever coming to court, a judge is murdered on Khagodis--the planet where the amphibious human slaves were first bred--and the man who first broke the slave ring must find a way to bring these villains to justice.