SandDepth calculates weight of sand needed for an aquarium to achieved a
desired depth.
You tell it the size of your aquarium by adjusting sliders and the desired
depth of sand.
It then calculates how much sand you need.  It works in metric or imperial
US measure.
You can also calculate how deep a given weight of sand sand will spread in
your aquarium.
As a side effect, it calculates how many US gallons/liters your tank
holds.
You can run it either as an Applet on the web, or as a desktop utility.
So you can add it to your aquarium website as a service to your visitors.
The logo is a pile of fine quartz sand.  It may not be obvious at a small size.