Originally Posted by Link119
Get pool filter sand instead of pet store sand because it doesnt float around if you swish it...
Doesn't mater what sand you get so long as you rinse it well first.
Place the sand in a bucket. Fill the bucket with water. Swish the sand really good until you have all of it moving around in the bucket. Pause a few seconds, and then pour out the water and whatever is still floating around. Repeat the process about 10 times to get all the "float" stuff out of the sand. You'll likely loose about 2-5% of your sand in the process. But once you are done, you will have sand that if swished around in the tank with instantly fall back to the bottom of the tank.
I've done this with pool filter sand, play sand, and ultra-fine sand from a fish store. All have worked the same. The only difference is that the finer sand is easier to make move. For example, I placed a air stone on the surface of the fine sand and got lot of sand raining down on decorations anywhere near the air stone. On the other hand, I've been able to place a bubble wand UNDER the pool-filter sand for an interesting "the ground is bubbling" effect without sand getting redistributed around the tank.
Now one issue you are likely to fact mixing rock and sand is that once mixed, you can't un-mix them without filtering the rock from the sand. The rock will tend to sink in the sand with any disturbance (like during a water change) so I would suggest that you have something like a sand side and a rock side to the tank. I've got a 20L tank that the front right corner is nothing but rock. That's where I do the water changes as the rock holds its place better than the sand when refilling the tank. Where the sand and rock meet, the two will blend together and change over time.