They might have been sick already. Also, the water can become cloudy if there is some outside element effecting your water. For example, if it was suddenly much hotter in the room where your snails were, there could be a sudden increase in bacteria and decomposed feces and food and plant life that created toxins in the water and thus the untimely death of some of your marine life - including snails. fish eek cool

Mary Brennecke Fish Site