Are two better than one - cannister filters - Page 2 - Tropical Fish Keeping - Aquarium fish care and resources
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post #11 of 11 Old 12-27-2009, 08:29 PM
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Assuming we are talking about a planted tank here, rsn48, then you will want minimal filtration and water movement. First, faster water flow through the tank is detrimental to plant growth (and many fish too) and there will always be a "battle" going on biologically, as I'll explain below. Second, you do not need biological filtration in a planted aquarium because that is exactly what the plants are doing, and not the bacteria.

The rate of water flow through the filter has an impact on the amount of oxygen drawn into the water, and carbon dioxide (CO2) expelled from the water in what we call the gaseous exchange. Surface disturbance speeds this up, as does higher flow filtration, airstones and bubble effects and powerheads. There are two detrimental issues to this: CO2 which is extremely important for plant growth is driven out of the water faster, and oxygen is brought into the water at levels beyond what is good for the plants. Plants have more difficulty assimilating nutrients when the oxygen level increases. But the more significant aspect is the CO2.

Submerged plants have difficulty obtaining enough CO2 in nature and in the aquarium; this fact is believed by many to be the reason for the inherently slow growth and low productivity of aquatic plants over terrestrial. Further, freshwater emerged plants have been shown to be more than four times more productive that submerged plants. The reason is because CO2 diffuses so slowly in water as opposed to air, and this limits the underwater plant's uptake of CO2 because the CO2 molecules don't contact the leaves quick enough to meet the plant's needs. Aquatic plants have to use enzymes to rapidly capture the CO2. When the CO2 levels in the water become depleted, these enzymes sit idle, so to speak, but the plant still has to provide energy to them. This results in a reduction in photosynthetic efficiency and therefore growth of the plant because energy is being wasted. Thus, any thing that removes CO2 in however small an amount will be detrimental to the plant's growth.

In a natural or low-tech system, the balance between the 17 nutrients (one of which is carbon) and light has to be there; so anything that may impact however slightly can become a critical factor in less success. The one thing we cannot "control" in this type of setup is the CO2, by which I mean that it is entirely dependent upon the fish and biological processes; with light we can control it, in intensity and duration, to balance, as we can with the other macro- and micro-nutrients through fertilization. Plants will photosynthesize up to the factor in least supply. Many have planted tanks that fail because the CO2 is the limiting factor, and algae will take over because it is better able to use carbonates for carbon than most (but not all) plants. The point here is that nothing should be allowed to negatively impact the CO2 in a natural planted aquarium.

The water flow is important for bringing nutrients to the leaves and roots, and keeping the leaves free of sediments. But there has to be a balance so as not to adversely affect the plants ability to assimilate nutrients including carbon from CO2.

Turning to the biological filtration point, in a well planted balanced aquarium the nitrification bacteria (nitrosomonas and nitrospira) are in very low numbers. The reason is, the plants use the ammonia/ammonium quickly, and little is left for bacteria. This is a domino effect; little ammonia and few nitrosomonas bacteria means little nitrite so few nitrospira bacteria, and finally low nitrates. Nitrates will frequently be below 10ppm in a planted tank; it is not because the plants are using the nitrates, although some do, but more it is because the plants prefer to use ammonium for their source of nitrogen and the nitrification cycle is minimal as a result.

The only purpose for a filter in a planted tank is to create an adequate water flow to benefit the plants in assimilating carbon and other nutrients, and to remove minute suspended particulate matter from the water column via the filter pads and media. This is mechanical filtration; chemical filtration is not recommended for planted tanks, and biological is basically unnecessary with respect to providing equipment.


Byron Hosking, BMus, MA
Vancouver, BC, Canada

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian. [unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]
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