John, The pair doesnt need to carry fry.. just the act of spawning is enough and even thats not necessary. We see alot of external bacterial problems with discus. It even happens when you mix stock from different suppliers..most of the time it clears up on its own or with salt added...
Nuking your sponges with pp will probably not make a difference here... just rinse them real good and it should be ok.
hth,
Al