02-19-2007, 12:20 PM
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I used sand as the bottom layer of substrate, followed by fluorite followed by gravel on two tanks. I wouldn't do that again - the sand becomes compacted, anaerobic pockets form, and toxins start leaking into the water. You have to stir the sand with a long probe. More trouble than it's worth.
Now, you can put a thin layer of gravel down on top of sand, no worries. You can also mix gravel into sand to get a substrate that has characteristics of both (I've got eco-complete mixed about 4:1 with gravel in one tank, and it works pretty well.)
The best way to mix sand and gravel is to have a beach area. The substrate for most of the tank is gravel, but using inert edging - either plastic or stone, separate out a portion of the floor of the tank and put the sand on that. Using plastic or pottery as the edging, you can spread a layer of aquarium sealant on it and stick small stones and gravel on it to make it blend in. If you do that, let it dry for at least 48 hours (72 is better) before adding water.
Something I am planning on doing in a sand bottomed tank that I want to have some deep rooted plants in: Cut the bottom off of a clay flowerpot. Put it cut side up (wide side down) on the floor of the tank. Fill it with gravel and plant that. You could also use PVC pipe as edger. If it's big enough, you could drill holes in the side facing out and the whole thing becomes a cave along the edge of your beach. Just make sure you'll never need to net any fish that likes to hide in there.