Planted Tank Enthusiast
Join Date: Jul 2014
Location: California for school, Arkansas for home
If you're talking about snail shells for decoration, then I suggest the shell-dwelling cichlids of Africa, or maybe bumblebee gobies with a tank that small.
If you're talking about keeping snails as the main residents...go for it! Depending on the species of snail, you can have anywhere from bullet-proof (bladder/pond, Nerites, MTS) to relatively sensitive (the rabbit/elephant snails of Sulawesi). With just a 12 liter/litre tank, I wouldn't go with rabbit snails (tylomelania I think), as they're just too big. There are other snails out there, like the "Golden Mystery" snails that I loved so much as a kid.
As for hardness, the snails prefer water with more calcium in it, that way it doesn't dissolve their shells. Acidic water will dissolve the shells. More snail shells will raise the hardness of the water, similar to how putting a bag of crushed oyster shells in the filter raises the hardness.Calcium carbonate substrate would be nice, but not a requirement. As always, research is key.
So many fish to keep, not enough aquaria.