How can you keep a big ray in a small tank and maintain quality water? You can't
When I first bought a ray I kept a motoro in a 75 gallon for 3 years. He never grew much ,his tank size was the reason. It was impossible to keep water good , so I beleive Also , that small volume of water is more likely to be the factor why they are not growing,but also I don't beleive if you did find a way to maintain water pristine that the ray would grow 20" in a 16" wide tank? Maybe I did see some hens in a pet shop 10 years ago here lol
change water change water.
that's all, change water.....even 95% twice or more per day....
the smaller the tank the more (and more often) you need to change water