Barbs in general are very sensitive to water parameters. 6 in a 20 gallon tank was pushing it and then adding 3 more was really pushing it. If you noticed their gills flared at all when you found them, that's a sign of lack of oxygen or water conditions going bad. If they died with their gill plates closed, that's a sign of a natural death. When you have 6 fish die at the same time, that's a sign of water issues. The fact that you did tests and they were " basically the same" says to me that it wasn't okay at some point and what the Barbs contributed to it not being okay was remedied by your biological filter or your filter that re-oxygenated the water. You would need to supply more specifics as to which fish died, the length of time from the addition of the new fish and your specific set up and water values before and after the additional fish and post deaths to make a more concise diagnosis.