My Joey gets them sometimes, like when it's moisty weather like you said. I've treated them successfully with a mix of Epsom salt and lukewarm water. I usually take a small plastic bag, big enough for the foot to fit in, it's easier to handle than a bowl.
I mix Epsom salt in the water till it doesn't dissolve anymore, so quite a lot. Then I dip the foot twice a day for as long as I can (cooperation issues) a few minutes are usually enough. It goes a way in a couple of days