Many of the American goldfish boards go for the 10gal to 1 goldfish rule. By that rule, ur redcap oranda is the only fish u will be able to keep in the Orb.

Be aware that Orandas can get big.... World Record Oranda, 'Bruce' was 2ft long! It's a half truth that fish will grow to the size of their confines. In a small tank, don't expect your fish to grow to the size of 'Bruce', but don't expect that the goldfish will stay at 2inches either. A 3-4inch goldfish within 2 years should be expected of.

If you want to add any more fish, I would suggest either 3-4 zebra danios or 3-4 white cloud minnows.