multicure (the one is have contains malachite green etc, not sure if yours is the same) hurt my betta's skin and made it really dry looking (i'm guessing it burnt away his slime coat). if i were you i'd put in a combination of stress coat and perhaps a gentle antibacterial- make sure its nothing harsh because big water changes are hard to do with fry. aquarium salt (non iodised, not table salt) is also a good preventative for fungus in the tank and is recommended by many as a general tonic for bettas in low concentrations.