03-06-2012, 08:06 PM
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There is some truth to what the store employee told you, but gravel can do the same if it is too deep and not properly maintained, so I wouldn't let this put you off sand. For years I used fine gravel, then two years ago began experimenting with sand and now love it. The trick is not to have it deep, in a 90g I would aim for 2 inches overall. I have this in my 115g and it has worked well.
If plants are intended, those with good roots systems like swords and crypts will help keep the sand loose and well oxygenated. And Malaysian Livebearing snails work wonders, as they burrow throughout the sand.
I would not use silica sand though, for two reasons. First, it is white and no fish likes a white substrate. Second, silica can cause diatoms, brown algae, which feed off silicates. The best and least expensive sand is common gray/tan play sand. I use Quikrete Play Sand from Home Depot or Lowe's. It took two bags for my 5-foot 115g, a total of $14. You can buy dark (black) sand for aquaria in fish stores but it will be considerably more expensive.
Photo below is one of my playsand tanks.