milligrams per liter (mg/L) and ppm (parts per million) mean the same thing as Graham replied.
Yes, Lynne, your point is correct. The pH of water varies significantly depending on sample history and test conditions.
I closed the bags as soon as we took out the fish until I could make the readings, in answer to your question. But it is definitely possible the pH was even lower in the bag before opening it, no argument.
And Graham's points about bacterial gill disease are also correct, in that chronic problem ammonia levels do lead to bacterial gill disease. The issue is how much ammonia how long will lead to bacterial gill disease on what variety of fish? Please note one of the references I cited above discusses that very issue for carp on page 2, post 19, of this thread. Except instead of using the general term bacterial gill disease they were more specific about gill symptoms with more scientific terms for the observations, as usual in technical publications.
Your koiphen chemist and environmental scientist.