I went out and bought some PE mysis after reading about them here. To be honest, my horses didn't take well to them at ALL. I just went back and got more Hikari after feeding PE for a week. All of my horses are adult-sized, and they even hunt saltwater ghost shrimp when I add them as a treat, but they were very picky about the PE brand. My H. erectus female actually spit the food back up on two occasions! I don't know why they had such a reaction, but I won't take chances.....Hikari mysis it is. They get live ghost shrimp as a treat once every week or so.