I have both a marine and freshwater tank. The freshwater tank is only 5 gallons, but I have an assortment of fish, algae cleaners (fish and snails), and some freshwater plants in a shallow colored artificial gravel. I keep finding these light-tan, coral-like stones in the bottom of my tank. They're crystalline hard, with cell-like structures, much like coral. I thought they were the skeleton or artifact of a dead fish or snail, but everything seems accounted for. I don't get them often, but lately I had two in less than a week. Any idea what these are, what causes them?