6 months is too long, in my opinion and experience. I made the exact same mistake with my very first 55 gallon tank and bought 6 bala sharks, 2 tinfoil barbs and a sailfin pleco. I swear it was only about 3 months before each of the very small bala sharks were over 6 inches long and the tinfoils were doubled in size. The pleco was by far the fastest growing and was such a strain on the bioload that I was having regular bacteria/algea blooms.
This Bala Shark topic is just a pet-peeve for me. I have no clue why they are so commonly found in LFS around the world and it always seems that no LFS employee cares about who they sell them to for whatever sized tank. These fish can be hurt be being kept in too small of a group and it usually results in stress-out balas that are open to Ich and other problems. We all realize that they can be bought small, but they are very fast growing fish and not even one is really appropriate for a 30 gallon tank.
Sorry that you got stuck in this situation. It is a common occurance so don't feel like you made the mistake. The LFS is at fault 100%. Like everyone points out, do some research and if you don't find the exact fish you want, then you will at least be 'armed' with the right questions to ask the next time you are thinking about a fish to purchase.
