depends on how long you plan to keep your arro. they get very long and need to turn around. arro's are a great choice. i have one arro in my ray tank. very conservitive and likes to be petted. kind of like a dog now. he's a little over a foot long. geo's are another great choice. however you'll want to grow them out to adult size as rays will eat them for dinner if they can. i have a friend in peru who, i am very disappointed to say, feeds his rays discus! i guess when you live there, they are common. makes me sad. so please don't keep discus in your tank. i kept butterfly fish in with the rays until they got eaten. please choose wisely my friend. and don't spend alot of $$$. until you are certain, they won't be eaten.