well if your problem is the big one picking on the small one it would make sense to remove him. Then again another one may become dominant and do the same thing. Although it may be better to buy more b/c they do better in larger groups maybe get 7-8 total and agression may calm down and equal out. Just keep an eye on them and see if it continues. I would add more though. Just make sure you have a tank big enough for all of them when they are larger, or when they are venting size then vent them out and keep a good male:female ratio and then sell the others that you dont plan to keep. For now I wouldnt worry though, its a new tank give them time. =]