It's the dirty little secret of the aquarium trade: You can get certain supplies and equipment cheaper at the hardware store than at a pet shop. Playground sand is a perfect example: If you us sand for your aquarium, play sand works fine, with a few caveats.
Most aquariums have pea-size aquarium gravel, because sand can have a few minor drawbacks. For example, water doesn't flow as easily through sand, so anaerobic pockets can form and harbor dangerous bacteria that produce chemicals dangerous to the health of your fish. On top of this, sand is more easily sucked into aquarium filters, where it can cause damage to your equipment.
When to Use It
Those drawbacks don't mean playground sand is not an option for any aquarium. If you have burrowing fish, sand is a suitable substrate. Burrowing fish will dig through the sand, preventing anaerobic pockets. Loaches and freshwater eels will dig in the substrate, hiding there between meals.
You may want to use sand if you have live aquarium plants. Like fish, different plant species have differing care requirements, and some prefer sandy substrate. If you use playground sand, include fertilizer -- either mix it with a fertilized aquarium substrate or add fertilizer tablets or sticks directly into the sand.
Preparing Your Sand
You have to wash playground sand before you add it to your aquarium to avoid clouding the water. This is the same procedure you follow with aquarium gravel. Put your sand into a bucket or bowl, then fill it with clean water. Stir the sand, and pour off the water. Repeat this until the water pours off clear. If you skip this step, the sand will cloud the aquarium water.