03-14-2010, 03:04 PM
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Substrate is very important for several reasons. Sand and gravel will both work, but I would recommend gravel for you, as I'll explain.
In a planted aquarium there is a significant biological process going on in the substrate. Aerobic and anaerobic bacteria live there and interact with the plant roots to break down organics into nutrients. The plant roots release oxygen to keep the aerobic bacteria alive. Water must be able to easily pass through the substrate as part of this, but not so easily that it affects it negatively.
A too dense substrate like sand has the risk of compacting because of the organics and plant roots. Gravel will also compact, but it is much less likely to do so. Which is why every plant authority I've read to date recommends gravel over sand, and some recommend soil with gravel on top, but that's another issue.
Fine gravel, grain size around 1-2 mm, works best. It is small enough to anchor the plant roots, allow good water passage, oxygen and gas exchanges, etc., and not too fine to compact quickly. Too large a gravel has the opposite effect, it allows too much water passage and bacteria don't have the best opportunity, plus plant roots do not fare as well.
You can buy gravel in bulk which is much less expensive. And in a dark natural colour. A darker substrate not only looks better, it is better for the fish that prefer dark substrates. Fish will be more at ease over a dark substrate and thus show their best colours. Black, dark brown, dark gray, or a natural mix works well.