I have two tiger sand conchs, two giant Florida fighting conchs and a queen conch. They stay under the sand most of the time. That's completely normal. They stay put in one place for extended periods of time if they have plenty of food available in that location. Out of 6 months I had the fighting conchs, I've seen them roughly 30 times. As for the tiger sand conchs, had one of them for 3 years and very seen it rarely. Only when they decide it's time to move to a different part of the tank. As long as you don't have any preditory snails like whelks or nassarius, they'll be just fine.