I know this must be frustrating for you, but it will get better
You may have never completely cycled your tank, if you are still getting ammonia readings. The problem is, your fish are probably suffering. If you can here are the steps I would take to put things right. It may seem like back-tracking, but it will make for a much healthier, happier tank.
1) I would take all of your fish back to the LFS.
2) Start feeding your tank either fish flakes or ammonia daily to get the tank through the cycle.
3) Once the tank is cycled and you have the following readings: 0 ammonia, 0 nitrite, 5-20 nitrate, you can add 2-3 fish.
4) Wait two weeks to avoid an ammonia spike and then add the remainder of the fish.
Remember: You only want to house a maximum of 10 inches of adult fish in your tank. Right now your tank is overstocked which is probably adding to the problem.
If you do not want to take the above steps and you can get your hands on some bio-spira, that will also get your tank through the cycle within a week. You should still take back a few fish though to avoid future problems. You are pretty much stocked with the 6 tetras
