Sorry to say this, but I'm still looking for at least one thing done right. You knew they were illegal, you knew that you didn't know how to care for them and you don't know what type it is (meaning you don't know how large it will get) - but you bought a pair. (Paid too much by the way). You acclimated them FAR too fast. The picture you posted shows a very weak and starving stingray pup. I'm guessing the other was in just as bad of shape. So that combined with shipping and fast acclimation killed it. You've got other fish picking on the one that is alive. So those need to be removed ASAP. Get some heathly food for it. Chopped earthworms usually does the trick. Get your temp into the low to mid 80F range. I keep my pups between 83-84F. Cut out half the lighting to reduce some of the stress it's going through right now.

