Spiked nitrates, nitrites, and ammonia? 5 Fish are glancing. I just lost my clown loach.?

I think I lost my loach because I treated the entire tank with quick cure rather than individual fish. I started out with about 12 fish that were glancing. My first treatment and 20% water change was last Sunday, then Monday, Tuesday, & Wednesday. I had no idea that quick cure depletes the oxygen in the water (clowns are very sensitive). This past month prior to treating with quick cure, I lost 2 pregnant female guppies and 1 female Sunburst platy. After the quick cure I lost my clown loach. 4 fish in 1 month. Needless to say a lot of crying. (yes I cry every time I loose a fish). I knew my sunburst female was sick. For 2 weeks I thought her behavior meant she was pregnant. I have a male sunburst as well. Then after further research I decided she may have had clamped fins. I treated her 3 times then on Sunday the whole tank with the quick cure. She died the next day. My loach died this morning. I have done 6 days of 20% water changes and I can't get my nitrate, nitrites, and ammonia levels normal. I have used almost 6 oz of prime. This afternoon, (Sunday - 7 days later) I noticed about 5 of my fish started glanicing again. Should I retreat my whole tank or just the fish. Also, prior to last weekend I have had heavy leaf loss on my live plants so I used aquarium safe liquid plant food for the first time in my aquarium. i use 1 tablespoon of tropical salt every water change as well as heavy vacuuming. I am at a loss of what to do. I have a new - pre-owned 55 gallon aquarium. How do I set the aquarium up and transfer the fish with all of these problems or is this the best time? This morning prior to my water change was the first morning my levels read perfect or normal and then 8 hours later they are spiked again. What is going on?