Treatment for Ammonia burn?

I work at a small pet store, and we get a large amount of feeders in weekly. When we get the feeders in, they are in a small amount of water, and the ammonia is at about 9 inside the bags of the goldfish. The water is bright yellow. It is awful. I get them in the tanks, and the temperature is about 66. None will die for about 3 to 5 days, then all of a sudden we will lose maybe 50 or so a day. The goldfish, and rosy red minnows both die a lot. Easily 40 to 50 a day. We have about 200 small goldfish and rosy reds in each tank and we have 3 tanks for small goldfish and another 3 for rosy reds, and 2 tanks for large goldfish. We have about 100 large gold fish in those two tanks. The water in our system is fine.The temperature is at about 66, and ammonia, nitrites, and nitrates are at 0. The PH is about 7.0. I have done research and am almost positive the goldfish are suffering from ammonia burn. I read that the symptoms of it do not set in for about 3 to 5 days. The goldfish are shipped overnight, and rarely is one dead. They almost always will all be alive when I receive them and will live for 4 days. I am starting to put water conditioner in the tanks, so I can help there slime coat. I am thinking that the slime coat is damaged, and could cause an infection since nothing is protecting them. Should I continue with the water conditioner? I checked and it does add layers to there slime coat. I just want to know what other things I can do to reverse the ammonia burn and stop any infection that could result from the ammonia burn. Would adding air pumps help? I know that the ammonia burns there gills, so it could be hard to take in oxygen, right? Any suggestions as to why they keep dying would be greatly appreciated. Any more information please let me know!