hi as said tiger barbs need to be in a group of at least 6 to be happy they are very active social fish not loners. i know when we put a molly juvenile of about 1 inch long in our main tank ( she was still quite narrow accrossways then) there was a few scary moments when our barbs noticed her & took a second look as in 'hmmm could i fit that in my mouth??? but she was ok & never got attacked, i dont know if id risk fry that size in the same tank as a bored single tiger barb might be too tempting to resist! esp if theres not many hiding places for them. could you not buy a simple small tank for your betta? ( & get your barb some buddies)