During those 6 days, what were you doing to introduce ammonia into the tank? Ammonia drops? If you just filled the tank and waited 6 days, then the cycle has not even started as there needs to be an ammonia source to get things rolling. Now that you have fish in the tank, for better or worse, THEY are your ammonia source and your tank will now begin to cycle. With 3 small fish in a tank that size, it may take a little time for the ammonia to begin showing up on your tests. Be patient, it will happen. Are you also testing for
nitrite? Sometime after your ammonia peaks, it will begin to come back down again as bacteria grows and converts it to nitrite. You will need to monitor the nitrite until it too is converted into nitrate by another form of bacteria. Then you will be left with ammonia and nitrite readings of 0, and some level of nitrate, which will indicate your tank is cycled and you can begin slowly adding fish. All of this could take several weeks, or even a couple of months. Now that fish are in the tank, you will need to test frequently and walk a fine line between allowing enough ammonia in the tank for the bacteria to thrive and grow, but not allowing so much that it kills the fish. Tiger barbs are fairly hardy and will be able to survive the ammonia better than some fish, but you will still want to do sufficient water changes to keep the ammonia from climbing above 1.0.
Hope that helps!