Live Rock, Live Sand, and Algae
Before you examine the various types of filters available, you should be aware of what might be called natural filtration systems. The term live rock refers to rocks that have been colonized by various algae, filter-feeding animals, bacteria, worms, sponges, and other flora and fauna. Even a very small piece of rock can harbor a multitude of such creatures.
Live rock provides a few benefits for the inhabitants of your tank. Some filtration is accomplished by the nitrifying bacteria inhabiting the rocks, while the larger species on the rocks assist by metabolizing the waste products of the other inhabitants of the aquarium and by consuming detritus and particulate matter. Some of the animals and plants on live rock may also reproduce in the aquarium, providing a supplementary food source for filter-feeding invertebrates.
Because live rock contains living organisms, it must be introduced carefully into the aquarium. You should test your water for ammonia for a period of time after the introduction of live rock, as tiny creatures may die within the rock and pollute your aquarium. The use of live rock has been most closely studied in marine aquariums, but the same basic principles apply to freshwater systems as well.
Only purchase live rock that has been commercially farmed. The collection of rock and coral is a serious environmental problem that threatens the survival of many species. Also, certain rocks can leach compounds or minerals that can quickly poison animals within the confines of an aquarium. This is less likely to occur if you use farmed rock.
Live sand is similar to live rock in that it is introduced into the aquarium with a full compliment of tiny creatures such as worms, crustaceans, and sand hoppers. Filtration systems that utilize live sand as the sole substrate for the aquarium are sometimes referred to as plenum systems. Occasionally, large outside filters are modified to contain a separate chamber for live sand or live rock. This somewhat lessens the amount of waste material that actually comes in contact with the sand or rock, but it does prevent the aquarium's larger inhabitants from consuming the tiny beneficial creatures that dwell in the sand.
Macroalgae, commonly but erroneously referred to as seaweed, utilizes nitrates, phosphates, and carbon dioxide in its metabolism. Healthy populations of various algae species can go a long way in improving water quality. An algae scrubber is a separate container to house the algae that is used for filtration. Water from the aquarium is directed through this chamber and back into the tank, much in the manner of any other filter. Whether grown in the aquarium itself or in a separate chamber, marine algae require a good deal of light at specific wavelengths. Be sure to purchase lamps specifically designed for growing such plants. Judicious pruning of the plants will encourage new growth and ensure that the uptake of nitrates and phosphates continues at a high level. In freshwater aquariums, plants and algae perform the same role as does marine macroalgae.
Natural filtration methods such as live rock, live sand, and algae are most effectively used as supplemental filtration systems. Although quite a few privately owned aquariums, and even some public aquariums, have been successful in relying almost entirely on natural filtration, such an undertaking is risky at best and should only be attempted by experienced aquarists or professionals. Your first efforts in this area should be with a very small number of hardy creatures in as large an aquarium as possible.