minimum tank size

Hi, my LFSs are a Petsmart and a Petco. The Petco had no tank size labels in front of its fish, but the Petsmart did. I Remember when I first walked in their and it said Oscar minimum tank size 40 Gallons, and I thought oooof. Bala Shark was 40 as well, Anglefish 20, Common Pleco 55, and Koi...