Help l Animal Cruetly l Need Answers?

Okay,my mom and dad bought fish and bought a tiger fish,suposly the tiger fish kills one fish so my mom calls the lady who selld it to them,wich is a pet smart worker,anyway we call her and she said flush the fish down the toliet which is dumb bc its illegal when its alive,so he does it and then i say its animal cruety so my parents say its just a fish,..well i wanna sue them for that poor fish that has not guilty for survieing ,so explain please,...This Fish Dusnt Deserve to Die a Slow and Painful Way :(