No, you dont need a filter as your LR
will take care of ammonia, but they can come in very handy for filtering out particles and what not. They also make running carbon and GFO
very easy. I use a Fluval 305. Keep in mind that youll need to clean these once a week else they become a little nitrate factory.
I bet your hermit molted. Mine do that all the time and look just like a dead hermit. Unless you see an empty shell nearby, assume its a molt. Last night I thought my favorite hermit was dying, so I made him a little house out of epoxy to keep him safe while he recovers only to find him WALKING around somewhere else!
Got me good!