Well, I am not sure about the shrimp and how they move around, if they stay on the bottom, I would gthink that they do, (aren't able to climb things) so I would bet that they would stay on each side with the bettas. As for snails, I think depending on the type of lid you have, they may be able to get over the divider or around it. I don't know for sure. You should maybe decide what type of snail you are thinking about, and look them up to find their behaviors and look into snails reproducing. I have heard that they can multiply extremely fast, and you don't want more than you can handle!!! On the divider, I found out friday that there are different sizes of holes in the craft canvas that is used when making it yourself, and the smaller the size the bigger the holes. I needed more circulation so I had to get a new sheet after researching it, I found that a 5 count is the biggest (smallest number) that is made. It works much much better than the 7 count I had in there... Let me know when you are ready to get your supplies if you want and I will help you with that. Also, the silicone that I used, is removable. I had to reglue mine... finding things out the hard way is what I am used to!!! How is Thanatos doing? I am sure he is getting prettier each day!