11-13-2010, 07:10 PM
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Plants will do best in a substrate of gravel with a grain size of 1-2 mm. It's hard to tell from the photo on that website, but this might be close or perhaps a bit larger. Strictly speaking, pea gravel is about the size of a green pea, which while it can work is not the best; smaller grain works better. I won't go into all the reasons why unless you ask. I have pea gravel in my present 70g Asian stream setup, and I have noticed it is not as good as the smaller gravels.
My other tanks hasve the smallest grain sized gravel I could find at the time. Some fish stores sell gravel in bulk, usually a natural (mixed something like the one shown from Lowe's) and sometimes a darker gravel. You will need about a pound per gallon, as you want 2-3 inches at the front and 4-5 inches at the back where the larger-rooted plants will be planted. If you go with the linked gravel, which comes in bags of half a cubic foot, I would say 4-5 bags; they say coverage is 6 square feet, but presumably that is just one layer, and you need several inches.
The other issue is to ensure this is inert gravel. Calcareous gravel will raise the hardness and pH, something you do not want in planted aquaria. There is no indication on the site as to what this is made of, and I wouldn't want to guess.