I am no expert on soil but I do know that potting soil has ferterlizer and other stuff in it. :shock: e also give the caged birds baths which they love :lol: but we use a regular kitty litter pan I mix half sand (from the creek) and half soil just from the yard, I watch were the wild birds bathe and I take the soil from there. They love bath time and it is the one time they all get along. It has become so popular that I have to put two kitty litter trays in the cages so they all can get a chance to waller in the dirt.
I would have never thought of using potting soil; one the expense and; two because I don't know exactly what is in potting soil. I use it for my plants but that is it.
I would take the birds to the vet and see if you have one in your area that handles birds we do not.
I realize this is no help, but keep me posted on how your birds are doing.