Leech means to release. The shells, like most things in the sea, have a coral basis. This means that they "leech" or release things that increase the General Hardness of the water, or make it Alkaline. This, in turn, tends to raise the pH in the water. Hard water is fine for saltwater fish, and some freshwater fish. But the fish that you have come from "old" water. This means the mountains and soil don't have much coral based stuff in them, if any at all, because they have been out of the ocean for a really long time. This means there is very little General Hardness, thus making the water soft, or acidic. These fish have evolved to cope and thrive in acidic water. This means that if you put them in hard, alkaline water, they don't do too well or even die. This is why you want to remove the shell and replace it with some kind of rock structure or driftwood. Also, don't use the white "holey rock." It comes from Texas, and it is almost pure coral. This stuff really raises the pH.
Oh, I almost forgot. There are actually several cichlids you can keep in ten gallons. Several apistogramas and killie fish (not quite cichlids) and some West African Dwarf Cichlids, like the kribs mentioned above, can be kept in ten gallons and, some, even bred! You really just have to research what one fit you water and likings.