I would take the infected frog out and make sure it is still eating like the other one. Petco's animals generally suck so chances are it was probably something that it came with and not something you caused. I would also switch to a moss and leaf subsrtate. I use that for all my frogs and have never had a problem. Orchid moss or spahgnum moss with a few inches of leaf litter works well and it will be softer on their feet and provide more hiding spots. I would also post this in amphibianforum.com if you don't get another response here.

The cut could be a lot of things, do you have pic?
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