A new CO2...what, tank? Did you not have CO2 running for awhile? CO2 lowers pH values and snails cannot maintain their shell integrity as well in lower pH causing shell erosion which is usually deadly to them. So if you didn't have CO2 for awhile snail population could increase then adding CO2 again caused a decrease.
Hard to ID moss in general but probably java moss. Small plant in the bottom right looks like a crypt of some kind and a bit of water wisteria, picture is cut off and bit fuzzy.
Hard to ID moss in general but probably java moss. Small plant in the bottom right looks like a crypt of some kind and a bit of water wisteria, picture is cut off and bit fuzzy.