Agree that looks like a bacterial issue. Those are usually caused by contact with a diseased animal or bacterial blooms due to overcrowding/poor husbandry.

IRT your test kits; they might be faulty. Only way to check that is to take water sample in a sandwich bag to local fish store and have them tested. Foam in the water can be caused by bacterial bloom or excess organics from the frog's condition.

You mentioned adding salt and some medication to the water. Can you please state the dose being used for both and how many days has treatment been ongoing? How long do you need to run that antibiotic? Myself would treat with a combo of Melafix and Pimafix; but you can't stop antibiotic treatments half way because that creates problems down the line with antibiotic resistant pathogen strains. Good luck !