Another betta gone ):

[Upirium]

This is ******** crazy and I don't know how this happened. Well, maybe I do...
I hadn't seen my betta in a few days. I was a little worried but I figured he was probably just hiding... Well today I finally decided to look, and I found him. Not all of him, just part of him.
Something ATE him. He was half a head, a heart, and some spine. That's all. What the HELL.
I think I know the fish that did it because its the only agressive fish in there. I'm going to get another betta, and when I do, mr meany pants is going with me. I can't take that chance.. I'm giving him to the pet store.

I don't know how I could have missed this though ):  

[Moth Feathers]

What aggressive fish do you have in there?  

[SmoothSavage]

It's always possible that he died and was then eaten as well.  

[Upirium]

Savage Destiny

It's always possible that he died and was then eaten as well.

I don't know how he would have died though. Both of the other fish are just fine, and I know bettas can be extremely hardy fish, if they can survive for a year in a shitty unfiltered, tiny bowl.  

[Upirium]

Moth Feathers

What aggressive fish do you have in there?

a baby chinese algae eater  

[Vanilla eXee]

They don't always survive for long periods of time in bad conditions. Only enough of them to perpetuate the myth that they can live in bowls.

Most of the time in community tanks, the fish dies first for whatever reason, other fish start nibbling, and then the owner thinks the other fish killed the dead one. I'd do a few things in your situation: QT your tank, minimum three weeks. Watch all of the other fish very closely during that time for any symptoms of illness or bullying. That means no new betta for that time period. If you don't have a water testing kit, take a sample of your water to the pet store for analysis. I know that the two big chains (PetSmart and PetCo) both do free testing, and most privately owned fish or pet stores will do the same. The last thing I would do is start doing frequent water changes, gotta keep the water clean. The solution to pollution is dilution. I can't tell you how many friends of mine have been in denial, when their fish start dying they will name any other reason in the book, but the fact of the matter is, they weren't keeping up on water changes and it was killing their fish.