12-21-2010, 01:21 PM
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The finer the grade of substrate, the more likely it will compact. That is the issue with sand. Depth also plays into this; the deeper the sand, the much more likely to compact.
I have sand in one (small) tank, I personally would never use it in larger tanks now that I have tried it. Small grain gravel (1-2 mm size) is the best plant substrate [except for those who advocate soil with gravel on top, another matter] biologically. Finding dark gravel is not always easy, depending where you live. In bulk it will be less expensive by far.
To explain the compaction issue a bit. The substrate is very important in planted tanks because there is a host of aerobic bacteria living there that work to convert organics into nutrients that the plants can assimilate. Aerobic bacteria needs oxygen, and while the plant roots produce some, it also comes down with the water. And the water is also necessary to dissolve nutrients, otherwise the plants can't use them. In nature, water percolates down through the substrate, the bacteria/biological processes cause it to heat slightly, and it then rises back into the water column. This continuous cycle is essential in planted tanks. If the substrate compacts, or is too fine for easy water circulation, this complex biological process will be hampered and sometimes stop altogether, causing a "dead" substrate and this is toxic. At the same time, the water circulation can;t bee too fast, or the processes can't function correctly and again the plants are prevented from assimilating nutrients.
The best substrate has a grain size of 1-2 mm and small gravel achieves this. I suspect this is why the enriched plant substrates are this size.