07-13-2009, 11:27 AM
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The answer to the question "are they necessary" is no. To explain: at the water surface there occurs a gaseous exchange, as CO2 (carbon dioxide, produced by the fish) is driven off and oxygen is absorbed into the water. The more surface disturbance there is (agitation of the surface) the more gas is exchanged. Same effect occurs from the constant bubbles from airstones and wands. This is bad in a planted tank where the plants need CO2 and this disturbance drives much of it off instead of allowing the plants to utilize it for photosynthesis; and this is important for the filtering ability of plants as I'll mention below.
As for the oxygen, a biologically balanced aquarium that is properly maintained (weekly partial water changes, adequate filtration, no overfeeding, stocking levels balance the water volume and chemistry) does not require anything further beyond the filter and biological cycle to remain healthy. Having live plants significantly increases the biological health of the aquarium; plants are nature's filters, and as Dr. Ted Coletti pointed out in his column in the July TFH, plants are the first and best filtration system in any aquarium, and far outperform any mechanical filter. Diana Walstad advocates a natural planted aquarium approach, and uses no mechanical filters. The plants are the only filters.