Hi,

the problem with pebbles is that dirt can easily be trapped inside the holes between them. Please make sure to siphon out as much as possible with each water change- your frog is still far away from perfect health and should be kept in a more or less clean environment. I don´t know the pellets you are using, but instructions on the packaging tend towards overfeeding, which should be avoided as well. I still recommend to feed every other day in order to give him time to digest properly.
Nitrite tests are pretty much a standard product, so they should be widely available. Do you know another store or would it be possible to order it online?
Fingers crossed that your froggy will get better soon