Conus striatus

Wow. I has been in Haus des Meeres (2021) and saw these small snails (lower row of your photo), with absolutely no signage like the one you show in the upper left of your photo, and with an appareance and shape of shell that made me think in olive snails (genus Oliva). Obviously they're "unwanted guests", in tanks intended for fishes (I remember it in a tank for Taenianotus triacanthus and another with Anemonia manjano, brittle stars and some other things I don't remember well, maybe mandarinfish). After asking here and in many other places with no result, I was doing my research I finally identified them, tentatively but with certain sensation of security, as a marginellid snail, Serrata fasciata.

But now, seeing your photo of what you interpret as juvenile cone snails with the signage intended for a cone snail in the same place, gives a radical turn to the general situation.

So, are you sure that these snails are young cones insead my previous ID?

Frankly, I don't know. Both the large proboscis (which was all I could see of the snail after numerous visits) and the small snails were in the same tank with the label shown on the first picture.

I think the greatest probability is that the large proboscis belongs to the cone snail while the small wandering snails were the supposed "accesory" Serrata fasciata. Juvenile cones should't have so oblong-shaped shells.

@Kakapo Could be. I'm definitely sure about the proboscis = Conus striatus.