06-15-2012, 11:31 AM
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The substrate depth depends somewhat on the intended plants. In a 29g you are not going to have the very large swords which have extensive root systems, so the substrate can be shallower. I have only about an inch in my 29g (photo below shows the good plant growth in this shallow a substrate), though I will admit I would have put in a bit more (around 1.5 inches) but I got tired of washing the sand.
I have 1.5 inches in my 20g and 33g, and about 2 inches overall in my 115g.
Having Malaysian Livebearing snails also helps as they burrow throughout the substrate keeping it from compacting by allowing water and oxygen to move freely throughout. Some "dead" spots are fine, in fact needed; one just doesn't want the entire substrate to compact which would kill plants and fish.