The thing about rays is yes a big footprint (8x4 minimum) is necessary, but you will notice they will be happier in a higher tank solely for the purposes of glass surfing. Remember these are river stingrays, and I would say 3 feet is a fairly safe depth for a river fish. I wouldn't dream of keeping more than a trio in an 8x4x2, once you see the size of mature females and especially after you have to separate any with a divider (due to aggression or pregnancy) you will start to realize that really isn't great room for them. An extra foot height only adds to sense of security and total size disproparotianley to the footprint, so 4 in an 8x4x3 is probably max for me. Picture living in your living room forever, wouldn't you want to jump off the walls for fun? Basically all your rays have is pounding bubbles at the top, deny them that due to a shallow tank and it may be harder for you to get them happy enough to breed.