A Possible Experimental Error

While there are several experimental errors that I know I have to consider when I am drawing conclusions from this experiment, one that I have to consider is not being able to properly identify dead brine shrimp. There are small "fluffy" white things floating around the bottom of my three tanks. I'm not sure if those are remainders of the several dead brine shrimp, extra bits of food, or excrement from the brine shrimp (that are still alive). However, in my first tank, to my knowledge, no brine shrimp have yet died, leaving my first theory about what the "fluffy" things could be, is wrong. Additionally, I had to change their food source for several days because we ran out of the brine shrimp food that I had been feeding my shrimp. I switched to feeding them yeast for 2-3 days. This may have effected them because of the change, even though yeast is an equally good food source for brine shrimp.