Snail shells in the aquascape
Snail shells. If your water is hard and gives a pH reading that's already too high, you won't want to compound your problems by decorating with snail shells. But in my very soft water, a large snail shell in a 10-gallon tank helps provide some desirable pH-stabilizing buffer. All mollusk shells are built of calcium carbonate, with a colorfully-patterned outer layer of organic materials — the periosteum — that protects the shell from acids. Be aware that if pH lies in the 6.0 to 6.5 range, you'll quickly lose this decorative outermost layer. The shell itself is made in two layers, with an upper layer of crystals of calcium carbonate packed together side-to-side standing on end, so that light is scattered, making this layer appear white, over an inner layer of "mother o' pearl," where the same calcium carbonate is laid in flat overlapping plates that refract light in iridescence.
Many small fish will take temporary refuge in a large empty snail shell. Minute Tanganyika Neolamprologus cichlids move right in and spawn in them. In my tanks, eroding snail shells change their appearance as they go. Of course you wouldn't want to use obviously marine shells or coral fragments in a freshwater tank. Stick to freshwater snails. The ones sold by a gourmet deli, intended for serving escargots, are just fine.