When you feed them shrimp with shell and FD krill, are they de-horned? I've heard a few cases where the horns on the shrimp heads can cause internal problems on fish. It can injure their digestive track and get lodged somewhere. That'd be the only suggestion I have for you homie.
The FD Krill I buy, at Eye Candy, comes without horns already. Maybe look for a similar type of hornless food? It might not be what happened, but it's a good thing to think about anyways.
***Just noticed the date on the original post....I'm a little late, haha!***