Initally I thought it would be b/c the eater is a female but if you have two males I am just not sure. You are feeding thawed bloodworms? Have you tried brine shrimp? Is the skinny one starving or just not as piggish as the other? Also, sometimes it has to do with how healthy they were and what they were fed at the lfs. Maybe the older one wasn't fed as properly and now he's just not a big eater??? Just really brainstorming here. I have had adfs that were never big eaters and some that were pigs, it could just be their personalities. Can you post a pic of them, esp the skinny one so the experts can see if he looks malnourished or just a tad thin? Also, how are your water params? Do you suspect any illness at all? How is the shedding? Did you QT the newer one before putting them together?

~Stephanie