Minimum Size
Everyone always asks the minimum size for lifetime. The smallest species will need 6x3, 8x3, or 8x4 depending on who you speak to. Either that, or a wide ponds atleast 6' in each direction. Please keep this in mind before purchasing your stingray, or have a good plan in place to find it a quality home. That means, within 3-5 years, you will need to purchase a 300-500 gallon tank for your new pet. Alot of people want to buy a stingray when they 'finally' upgrade to 125-150g tank, but don't keep in mind that there will be another upgrade right around the corner. Optimally, most common species of stingrays would do best in tanks with dimensions such as 8x4x4, 10x4, 12x4, and or a pond of 'mega-tank' of larger scale. This is reality folks, these are LARGE ANIMALS, not just fish.
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I think you should definitely add some cichlids. Uaru, Geos, or other more peaceful species would be great. Doing a planted setup would also be awesome.
I think you're on the right track. For it to be "entertaining" you want a good bit of movement and lots of different colors.
I would not bother with rays. Sure you could "squeeze" retics/teacups in there. Retic/Teacup's aren't the greatest species anyways. You'd just be better off sticking on the track you're on.