Is it normal behavior for Clarkii?

I have bought 2 clarkii clownfishes from lfs yesterday. They both are fine until at night. I notice that one of the clownfish's tail has been split into two. I was worry then I checked online and the tail should be fix in couple of days so I felt ease at that moment. Later on, when I went to sleep last night, I found my clownfish has died and the whole tail is gone. I found my two crabs are trapping the fish and eating the tail. Is that normal for hermit crab?. Is that mean other clownfish is aggressive enough to kill it? Another thing, In the morning I fed this other fish is everything fine. he was swimming around the tank. Then after I went home from class i want to give food to clownfish, it was hiding between the rocks. So i tried to turn off the light and only turn on the blue light to "calm" it and it works. I gave the food while the room is dark with blue light. It came out swimming and eating the food. Then the clownfish is on the sand and still alive. Is that normal for everyone? Do I need to do something else? The test everything is 0 and no nitrite, no ammonia, and no nitrate. The PH is 8.0 which is normal.