Their odds increase with added structure, but they would still be safest in a net. What I would do, if you have a decent sized net, is simply net as many as you can, prop that net at the top of the tank, and throw some of that hornwort in with em. Maybe catch half of them, and see which survive better (netted or free). Also, keep the adults well fed. If they are full, they will be less likely to actively seek out the young.