Extremely sorry to hear about your loss... I felt so bad from having pets die that I vowed never to get one ever again at one point, until I got my two RES. I'm willing to bet that it passed away and then the filter sucked it towards it, though, unless you had some sort of very powerful filter without a screen? Regardless, my heartfelt sympathies go out to you.

My smallest one actually hangs out by the filter nozzle intentionally, so maybe you can feel a bit better in the knowledge that it probably wasn't that...