In the past few months, several of the fish I've added to my tank have
died soon afterwards. They are bought from a very reliable LFS and I am
certain it is not that they are unhealthy or diseased when I get them.

The most recent victims have been a beta and an otocinclus.

The water is fine and there are a number of healthy, active fish - both
old and new - so we suspect they're being attacked by something else.

I don't think any of the other fish in the aquarium are of a violent
tendency. Currently there are mollies, ancistrus, one surviving
otocinclus and clown loaches. It doesn't seem likely that any of these
would be the culprits. There are also three shrimp, one of which has
grown quite large (about 1.5 inches long); I note that if there is a
dead fish in the tank, the shrimp are very quick to get to it - but are
they likely to attack a live fish?

--
FishNoob