Fish Killer

Question
Ok, I have a fish in my tank that is killing or eating other fish. The fish in the tank our not know for killing fish the size of the missing and dead fish. I have a mild behaved Betta, a temper mental red Tailed shark, a Corydora, a Plecostomus who's really big... guppies and I've added 2 algae eating shrimp (gone missing), 2 pea puffers (one missing) and mixed community fishes (not sure the breeds they were label mixed) The last three types are new... but the guppies have been turning up dead or missing for a long time now. I'm hoping that I can pin down the potential killer, and relocate him to another tank. Please if you have any though, it would help. All of the fishes are too big for the betta to be the one killing them.
Please help!
Trista

Answer
Hi Trista
Hmmm.....How large is your tank?  A lot of times aggression comes about when the tank is too small for all the fish, you get into territory issues.  I think you have a typo/cut/paste error(I do it all the time lol) did you say you're finding some bodies and some fish are just totally disappearing?

One thing I've heard a lot about bettas, is they can be ok in a community tank, but it's not recommended to keep them with fish that have colorful or long tails-like guppies.  Even female guppies now can have colorful long tails.  That can make them aggressive, like if there was another male betta in the tank.  Also dwarf puffers are known to be fin nippers.  Not a good community tank fish.  
Red tail sharks can become pretty territorial with other fish as well.  

What could've happened, the shark or the betta might've killed some of the fish or just harassed them enough that they died, and fish are known to be cannibalistic-especially on already dead fish.  The pleco could've very well contributed to that but sucking/eating the dead fish.  I'm sure there's bones somewhere in there, they just may have gotten mixed around.  The shrimp, I've put ghost shrimp in my tank for my dwarf puffers to eat before.

My guess would be the betta or the shark are the culprits.  Again, depending on your tank size, I'd move the betta and the puffer but not together in the same tank.  Add 2 more of the same type of cory cats-they like to be in groups of at least 3.  I have a red tail shark in a community tank, he chases other fish when they're in his area, but doesn't attack them or kill them.  You could relocate him as well, again I wouldn't put him with the betta or puffer though.  If you're seeing him going after the other fish, then he might be your problem then.  Like I said, they can get territorial.  

Hope that helps!  And good luck!!

Christy