becoming encrusted with coralline algae, would be considered “live rock”, and if not, is their any reason they couldn perform the same function in a marine aquarium?
< There is not restriction on the collection of empty shells, and unless you are intentionally collecting them with intent to sell, there should be no problem. In small numbers, the shells can be a beautiful addition to an aquarium. Unfortunately, the stagnant water inside of the shells may cause problems in the long run, especially if there are too many shells present in too small of a body of water. The "live rock " has much more surface area, and can therefore house more bacteria for more biological filtration. The surface of a shell cannot compare in that aspect. Piles upon piles of shells would also become a sink or deposit of detritus and mulm over time, thereby even further increasing the risk of oxygen depletion. One or two would not be a problem. Ten or twenty would. > I’ve attached a picture of a horse conch shell I found a few weeks ago off Florida’s gulf coast. If you look closely, you can see (in addition to the gorgeous purple algae) two fan worms (I think) on the bottom
