the lfs salesman was just that a salesman, his entire goal was to sell you something, if he told you the real answer to your problems was that you needed to feed a quality diet, and change more water do you think you would be leaving the store with anything besides a smile and your money in your pocket? api master test kit is a tried and true quality test kit, the nitrate test if i remember correctly is a p.i.t.a. are you sure your doing it 100% correctly? are you doing it exactly per the instructions? i believe you have to do something like shake the test tube for 5 minutes which sucks.
i did it so many times i have it memorized