First, jealous of you living in tropics. If I could keep fish outside year around it would get crazy fast.
Secondly, with rays the water quality is the biggest issue and pretty much any ray will eventually outgrow that system. Could it be done with the right filtration and lots of big water changes, yes. I would go with fish that look good viewed from above like rays, silver dollars might be nearly impossible to see. Go with a male ray, grow slower and stay smaller than females. Read all the ray stickies too it should help.
If I were you i would go much bigger with the pond and set up a ray community or a huge african cichlid pond. Once again, so jealous.
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