04-15-2011, 02:27 PM
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AbbeysDad has hit the nail on the head. If I may, I will just expand a bit, as I know there is conflicting information floating around on water changes with planted tanks, and some of it is not entirely accurate.
If you check into the Walstad or Barr methods, they recommend few to no water changes in low-tech ("natural") planted aquaria. I myself had a good discussion with Tom on this very subject, and we have differing views (which is fine). But something we all agree on (including Ms. Walstad) is that the fish load must be moderate and the plant load fairly heavy.
In a "perfect" system, with few fish and well planted, in sufficient volume of water, the plants could handle most of the "stuff" that we would otherwise remove with a water change. Nitrates is only one aspect of this, and in planted tanks the least important since the plants in a ":natural" setup ensure nitrates are minimal. There is more importantly what one author called "crud" that includes not only the dissolved waste and urine but also pheromones and such chemicals released by fish (and perhaps some plants, thinking of allelopathy or allelochemicals released by plants). It is believed by some that plants can handle all this, but the fish load must be proportionally low.
Most of us prefer stocking more fish in an aquarium. A group of 8 neon tetra in a thickly-planted 55g tank was one author's estimate of the maximum fish load for this to work. I'm not suggesting this is or isn't accurate, but it is food for thought.
The safer approach for our fish is to do a regular partial water change. As the prior member very correctly pointed out, in nature fish are always in "fresh" water. The ratio of fish to water volume itself is so enormous by comparison to the largest aquarium that we cannot even begin to come close. The only way to dilute all this "crud" is to remove it with a water change. No filtration system will duplicate this.
So now we come to the substrate. In any substrate in a planted tank, organics (the waste) gets broken down by bacteria in co-operation with the plants. There is a complex biological process involving the water flow through the substrate, aerobic and anaerobic bacteria, and plant roots. This occurs in any healthy substrate.
A substrate that releases nutrients should be an advantage to this biological process, but this does not directly affect the need for water changes. Only the number of fish--and the type/size--in relation to the number and type of plants and the volume of water really affects the frequency of water changes. The more plants and fewer fish, the less water needs to be changed to maintain a certain level of health. But the affect of water changes on fish behaviours and health cannot be denied, and this alone is positive proof of the benefit of any water change, and the more the better.
I change 50% of my tanks every week, and have done for more than 15 years. I have gravel in some tanks, sand in others, and Flourite in one. I am not aware that this in any way impedes plant growth. But I am aware without any question that it improves fish health. And that is the important issue.