no expert on diseases so looking through my health book the only thing i can find that might cause it is haemorrhagic septicaemia what i'm thinking is that this disease causes redding and blood stained fluid in the interanal organs in fish mabe it pucking out the fish and its staained from the blood its a type of bacterial ucler the treatment if the fish are eating and teat all fish in the tank is to use a antibotic medicated food and add salt to the water to ease osmotic stress (makes it easier to breath) its just an idea so check more into it first because treating a fish thats not sick can do more damage then good hope this helps !
