I did that and my clownfish and angelfish thought I was feeding crap or something. Everytime I thawed it it was like (Insert something everyone loves and is addicted to here) for them. They gobbled it up. Maybe just get a small cup, fill it with water from your tank, put the mysis in it and let it sit for 15 minutes in the fridge ten feed it, then put it back in the fridge for the next feeding. Repeat. Worked for me. Don't worry about the water in the cup and putting it in your aquarium because mysis is 80 percent water so it doesn't pollute the water as much as a pellet or flake. Don't feed to much because of that info, feed normally. Be warned if you do leave the cup out for more than 5-10 minutes, throw it out because the mysis will turn bad. Hopefully this is why this is the problem.