Unless the female was significantly smaller than the male I doubt that he would eat her without a trace of evidence. You should fine some part of her body if he did kill her, which may be a possibility. You should also check under and behind the tank decorations. If she was chased, the female could have lodged herself in a tight place to get away from the male. Another place to check would be behind your tank. If she jumped out, she could very well be behind your tank. I hope that she is just hiding.