The first question is whether or not they're the same species - most "mystery snails" sold in pet stores are pomacea bridgesii (brigs) though sometimes gold pomacea canaliculata (canas) will get mixed in with them. Check the link in my sources for good comparative pictures of the different species. The black snail is most likely a brig, though you may still want to take a look at Japanese trapdoor snails to make sure one of them didn't sneak in there.
Brigs generally mature at about golf ball size, and will need a good variety of blanched vegetables, snail cookies, and as much calcium as you can cram in their diets in order to be healthy enough to breed, as well as produce healthy young with good shells. These are not hermaphroditic like physids or similar types of snails, so next you have to figure out the gender of the snails you have. Hold each snail upside down with its trap door facing you, and lure the snail out with a bit of snail cookie or some greens. Try to get it to stretch its head out as far as you possibly can, then look to the right side on the inside of its shell. If it's a male you'll see a penis, and a female will just have a hole. The left side of the inside of the shell will always have the siphon, so make sure you're looking on the correct side.
pets question and answers,www.5d2d.com Next, lower the water level about 2-3 "from the top of the aquarium, giving them ample room to lay their eggs. If you don't have room for hundreds of baby snails, you may want to scrape off part of the egg clutch once it turns salmon-colored and only keep a small portion of it. When the eggs turn kind of papery and grey, the babies are ready to come out - you can let them hatch on their own, or if it's getting really dry and a lot haven't come out yet then you can gently crumble the eggs and drop the babies into the water. My babies have always loved fresh romaine lettuce the best as a starter food.
