Disclaimer: Regarding the tools and equipment I used in this process: I have used them in a manner inconsistent with the manufacturer's intended use. As a result, naturally, the risk of injury involved in their use is increased. If you choose to follow this process, please understand that you do so at your own risk.
Feather rock (FR) is a type of lava rock. Kind of like a volcanic glass. It can be very sharp. While working with this rock, it is possible to get a "splinter" from it. While using power tools w/ it, some of the material may go flying through the air. The use of safety glasses is strongly recomended. $10 spent on safety glasses is cheaper than having a splinter pulled out of you eye at the ER, I know this from personal experience. Gloves will protect your hands from cuts, scrapes and the general sharpness of the rock while handling it.
FR can be bought at most landscaping stores or at stores like Lowes or Home Depot. Most rock is sold by the pound. FR comes in a couple of different colors, as leaset that I have seen. Some of it might also have a vein of a different type of rock in it as well. It would probably be a good idea to test a piece of the rock before putting it in your tank. FR can probably hold a pound or two of water, so buy it when it has been dry for a few days.
Some of the tools that you will need are: A saw of some sort. I have used a bow saw (tree trimming type), a used 1 1/2" band saw blade about 18" long or a long saw blade of some kind. A drill with carbide tipped concrete bits, I have even used old wood "spade" type bits. Optional tools can include an ice pick, scratch awl or possibly even a long nail, a flat or round file, large and small diameter bolts (used to open up and shape holes). Gloves will protect your hands. Wear old clothes and work outside. Please keep in mind that some of these tools were not meant for cutting or cutting into rock. Using them in this manner will either dull them significantly or possibly even ruin them.