08-15-2010, 01:21 PM
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Plants will grow in any substrate, although the rate of growth can differ depending. It's up to you what you use. I always suggest plain aquarium gravel to those starting out with planted tanks because it is easier, there is less chance of trouble, and it does work. I'll mention the alternatives momentarily.
Regular aquarium gravel is inert (doesn't contain calcareous substances like limestone, dolomite, coral, marble that will raise hardness and pH). Get the smallest grain size you can, 1-2 mm is best. And in a dark colour. Black, dark brown, natural (the beige/black/brown mix) all work. This is important for two reasons; first, it calms the fish because it is "natural" and most fish we keep in planted tanks are used to something very dark below them. Second, the dark colour shows off the colours of the fish and plants. Fish will intensify their colouration over a dark substrate, Weitzman proved this. A 2-3 inch layer of gravel is fine; less at the front, more (deeper) at the back where the larger-rooted plants will be.
Now, you mentioned soil. A layer of pure clean soil such as you buy (not from the garden) with absolutely no additives like fertilizers which will cause problems. Over this a layer of gravel, same as what I suggested above. The problems with soil are 1) mess--if you are not careful you will have soil particles throughout the tank. Moving plants is tricky because this disturbs the soil and up it comes. If you have fish that dig they will be in the soil. 2) It takes a few months for the tank to establish itself with soil due to the release of CO2 and nutrients. During that time you can have issues with water parameters, conditions and algae. All this is in Diana Walstad's book, and her more recent article in TFH last year, so it is not my dreaming. She warns that soil has these drawbacks. But it works if you are prepared to deal with them.
For someone starting their first planted tank, I would rather you had good success than problems, so you don't get discouraged. Planted tanks are easy; I always say it is easier to handle plants than fish, and I believe it is. Keeping it simple means more chance of success. Once you have the easy approach mastered, you can move on to experiment with other substrates. Sort of like the very wise adage, you have to learn how to walk before you can run.