To me it looks like some sort of trauma occurred to injure the leg. It is possible that one of his tankmates may have bitten him over food.
What size tank do you have them in?

Are there any sharp decorations that he may have rubbed against/gotten scratched by/ etc?

A blister is fluid building up between layers of skin, usually from an injury or possibly a burn. Do you have a heater in your tank?

I don't see any blood under the skin, so that's a good sign. Is the frog acting like it hurts? (I know this is a loose description but favoring the other leg, acting funny, etc.?)

Do you think it is more like a fungus/infection?? I don't think ADFs can get Ich...but they can get a fungus called Chytrid in which case I would isolate the frog.

I don't think it would hurt to do a slightly salty water bath. I am copying the following from another thread:

"I would set up another tank - about 10+ gallons for the frog and use a tupperware to administer salt baths.

Use Epsom salt, 1/2 TEAspoon per gallon of water. Remember to treat with conditioner like you do for your tank during water changes. Leave the frog in the bath for up to one hour daily.

Put the frog in the QT (10 + gallon tank mentioned earlier). Do not put the salt bath water in the tank."



Sorry for the lengthy/all over the place reply, just trying to throw some ideas out there of what it may be.