Is it the 30 litre one or the 60 litre one? TBH (and I know this is bad news), I don't think the 30 litre one is really suitable for any fish. If you had a heater then you could possible squeeze a betta in but they do like access to a large surface area so it wouldn't be that happy. Is there any chance you could swap the biorb for a Juwel Rio or something else a more traditional shape?
Sorry, I know that wasn't the answer you were looking for These things are not designed with fish in mind, unfortunately. The RSPCA did a press statement about fashion tanks a couple of years ago here.
If you can get a small heater, have a look at cherry shrimp or snails. Believe it it not snails are quite interesting! I believe a single betta would be fine in one of those tanks.
If you are keeping it cold, goldfish are a definite no-no, as they grow quite large. I'm really sorry to say it but there isn't much you can keep in cold water in one of those. They would be fine as babies but will become stunted and sick as they grow inevitably larger