This has the makings of another of those reefing rumours. Let's get this straight.
From what I've read, over 100 species of "conch" have been described. A similar number of whelks have been described. In addition, there are a number of other gastropods that resemble these two groups fairly closely.
Collectors are fairly fond of throwing "miscellaneous" inverts into orders,
supposedly without the permission of the shops. Regardless, they seem to arrive here in significant quantities, sometimes in certain stores more than others. We know that generally stores do not take the time to identify these animals and check up on their diet.
The conchs we actually want are typically Strombus gigas (Queen conch) and Strombus alatus (Florida fighting conch). Although there is a dearth of information on the web, the experience of anyone I know of who has confirmed that they own either of these is that they are strict non-predatory omnivores (detritus eaters), or at least, do not predate anything we know of in the aquarium. I had a Strombus luhuanus (strawberry conch) and have no reason to believe it ate anything but dirt.
On the other hand, I've seen photos of many gastropods on RC and RDO that were reported to attack other invertebrates and cnidaria, and although I am not an expert, comparisons with photos of the above-listed species on reputable sites indicated that these were species other than those typically accepted as suitable.
Therefore I would caution anyone against saying "my conch ate my..." It is possible to positively identify many of these animals based on images and reports on the web, and I feel this would be a worthwhile endeavour for anyone owning one of these animals who is worried about what it might do. Obviously this is not an exact measure but it's better than perhaps proliferating the idea that any animal labeled "conch" is either a time bomb or on the flip side is never going to cause any problems whatsoever. Perhaps even more importantly, I feel that shops should take it upon themselves to attempt to identify species they receive, willingly or not, in order to protect their customers' tanks. When was the last time any of us saw an animal in a store labeled "unsuitable for reef tanks"...
http://data.acnatsci.org/conchnet/
http://www.gastropods.com/