I forgot to mention: sirens will take in small pieces of gravel with their food; they usually spit them out, but they probably do end up swallowing a few. I don't know if sand would be more or less problematic than gravel in this respect. I just sink a shallow watchglass in the tank and put the bloodworms in it with a turkey baster. The siren scatters some up into the water column, but the killifish I keep with him eats all those.