I cant claim that my water is paticularly good, I suspect that the snails where already pregnant when I collected them. But at least they havent died in the time I have had them nor have have they wondered out of the tanks which they can easily do. Even the nerites that I converted from salt/ brackish to fresh have stayed put in my open topped tanks.
I am having a bit of a hard job at the moment with all these babies, since we had to dismantle one of the tanks to catchout all the subadult bristlenoses, that meant removing all the plants, timber and ornaments. Now the tubs that held the various things I took out of the tank I am having to go through to find any wayward baby snails. So far I have found 4 and put them back into the tank. My husband has trouble telling the baby notopala snails apart from baby ramshorns so it looks like I am going to have to be the sole snail finder.