My fish in my tank are vanishing....?

I have a 20 gallon tank,temp 86-90..I have two plecs one 4 inch one 6 inches who have been the only two in there for a yr.I just bought 4 neons, 2tetras blk skirts,2 rasboras,3 guppies.2 male one female and I had 0ne female and two young guppies from the night before. I brought the fish home floated them in a bag till temps were the same. bought fish on the 24th and its the 29th.I have only two blk skirts 2 mollies,male and female and one female guppy.The rasbora dissapeared last night. I have found whats left of fish carcuses all chewed and barley any left.I qurentined the two mollies thinking they were the culprit but the rasbora still vanished.So is it the plecs? or the two blk skirt or the one female guppy.I even had tiny babies that vanished...Anyone have any idea????