More info though please. Are they rat pinks simply disappearing (in other words they've been eaten) or are they just laying there dead (some must be since you mentioned the dead ones had milkbands) or are they dead and partially eaten? How many females in the tub? How many total rats in the tub? Are both litters that were lost from the same tub? Any changes in husbandry, food, new pets in your house, anything at all like that? Any new rats added to your colony over the past couple of months?