Anything surface swimming would be fine. Kribs rarely leave the bottom of the tank so you could get a personable surface fish without interfering with their territory or breeding space. It might be difficult to find the right one since your tank is only 12" tall. Just be aware that keeping and breeding kribs in a community will likely reduce the number of fry that survive. Also, breeding kribs can be very aggressive, so you wouldnt want something with fins that would be damaged or that would fight back. I lost a swordtail to my female when she was guarding her fry. The swordtail just wanted to eat them all and was ignoring the krib's onslaught. She was pretty damaged before I saw what was going on and unfortunately she didnt make it.
