The biggest tank you can get the better. If you can't get something 240 or larger really should not keep rays. Honestly the larger front to back the better and min of 5 feet long. Motoro is easy but get huge huge. Marble might be better to think about as they stay a bit smaller. If you could find some hystrix they also would be a good option. Tank no matter what needs to be 24" front to back 18" will not do. 24 with rays is even small though.
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