Why wouldn't it help? Tetracycline is frequently sold as a fish antibiotic, and is also effective against myco. This might have been one of the rare occasions where the pet store advice was good.

Another possibility is to give them echinacea.

Something to consider is if it is worth treating your rats or not. You want healthy genes to make up your feeder colonies. While treating them may get them past this myco flare-up, it won't eliminate weak genes.

Here are some links from threads I posted way back when I was getting my mouse colony started and having all sorts of health issues with them.

This one is about the myco flare-up my first mice had when I first got them. I treated it with echinacea:
http://www.ball-pythons.net/forums/s...ight=echinacea

This one is about when I had some mice prolapsing, and eventually came to the conclusion it was a bacterial infection causing it. I treated them with tetracycline:
http://www.ball-pythons.net/forums/s...t=tetracycline