Fish Tank Dealers Fish Dying Fast...all Gone...now What?

Fish dying fast...all gone...now what? - fish tank dealers

I have a 6-gallon tank of freshwater fish at 8 on it for 7 months. One child died in the Neon. Some are bloody stress. I thought they drank it b / c they were (food malnourished once a day, sometimes twice). Often, salt. I made a difference of 20% water and have bought 3, more fish, then suddenly began to die my fish. Nitrates are high, and I lost 3 fish. My dealer (Petco) told me, not on the scales, which feed the blood worms and brine shrimp or a little food. He tried not to eat the fish. I have no other changes in the nitrate-normal water. So I missed the last 3rd All this happened within a week. I tested the water with the band after his death. Nitrates are safe, but the pH and ammonia is too low. What have I done wrong? I feel confused information from Petco all the time! Now that my tank has no fish, what should I do? The water flows. I have to wait for a change of 80%, 4-6 days, add fish and 2-3 again?