Barbs, usually tigers or greens, are most available. They are very active, and need to be in a school. They should be good with standard tropical flakes, and would enjoy frozen or freeze dried foods, too. If I remember rightly, cherry barbs are smaller and less aggressive. If you do get a school of barbs, keep a close eye on your betta to be sure he's not getting shredded, since a school doesn't always keep barbs occupied with each other.