Man i had the same problems with my marbles about 8-9 months ago. Had 9 mm in with a leo and 2 reg. motoros. All went through 5 weeks of qt and eating crawlers and shrimp fine. one day 2 if the mm stopped eating and lost appitie. within 2 days all the mm started breathing super heavy and stopped eating. leo and reg motoro still ate. up till that pointy i was doing 50% wc twice a day. within 3 days i lost 9 marbles. Till this day i dont know what the helll happened. I was feeding alot of crawlers and the 3 canister filters i had runnung were real dirty i guess. some people told me that cause of the filters the rays got a fungus or something. but no visible signs of anything external. Just the same mo. breathing heavy, stopped eating, and dead. Ive heard alot of people losing random mm the same way. healthy one day, dead the next. I just lost a nice pair of mm bout a month ago out of no where. mm curse i guess. im swithching to pearls. good luck bud