Ah, I thought about the exact same thing, the oysters will starve out, I had 2, a big one 4 inches, and a little one 2 inches, they didn't make it. I believe they need to live in green water to sustain their body mass. our tanks are just too clean. I saw an educational video online showing how much water they filter per day, 50 gallons. If I didn't use dino X at the time, I would've eaten them.
I do have a small clam that's still alive, it's 2 inches in size, he's the only one I didn't eat last summer, probably a little more durable than oysters. it's still going and pretty active, moving around in the sand bed. As for spawn, then you should try either sexy shrimps, or even feeder shrimps. Snails! They spawn regularly in tank, and are much easier to maintain.