The other snail is a species I have yet to own/positively identify. @ISpeakForTheSeas might know more.
Unfortunately, I'm only familiar with a handful of
Nassarius spp. at this point (there are so many species in the genus that it's a bit daunting to want to start looking into them seriously), but I can say that I suspect this snail is a different species than the Petco snail above:
The closest I know to this snail at the moment is
Nassarius graphiterus, and I'm not super confident that it's the same species (again, tons of species in the genus). However, as mentioned, the vast majority of Nassarius snails are scavengers/opportunistic predators rather than true predators (and even the ones I've seen deemed "true predators" don't seem to actually be very highly predatory; they seem quite lazy about getting their prey), and these are not an exception.
The Petco snail above I've seen most commonly called Tongan Nassarius and referred to as
Nassarius distortus; given the enormous variation between specimens that I've seen under both of these names, I'm hesitant to accept them all as the same species.
Even the ones that I've seen with the most evidence of predation (which I'd recommend as sump species) still seemed to either only "actively" (in quotes because I can't confirm how healthy the prey were before they were caught) hunt worms or vulnerable prey (such as other, smaller snails or moribund inverts), and these likely still prefer dead things.
No Nassarius species should be (I'm not even sure they'd be capable of) going around hunting healthy fish; it would be a very high risk activity with a very low probability of success, and most predation in the wild is basically about weighing risk/reward and the odds of succeeding.
With proper food availability, I doubt many of even the more predatory ones would choose to hunt instead of just eat some pellets/frozen/etc.; without proper food availability, I'd expect to see them going after other snails/each other before they starve, as the vast majority of snails (including Nassarius snails) just aren't equipped to take out anything other than a snail or bivalve.
All of that said, I'd consider things like the fish's age, disease, diet, etc. as the culprit here instead of the snail.
TLDR; The fish was most likely either dead or dying, and the snails just scavenged it - it's incredibly unlikely they would try to or be capable of taking out a healthy fish.
