a few larger rocks can also work well with fry.
if you plan on using rocks found from outside, first put it into a container with vinegar, if you see bubbles then this is best kept as a "pet rock"
and kept away from your aquarium.
i would NOT suggest boiling rocks to clean them, they can EXPLODE
due to water and air trapped inside them. (there was a sticky thread on here somewhere actually)
you can use an old toothbrush with just water to scrub them and then soak them for a few days in a seperate container or use a 1:20 bleach/water mixture to scrub and soak your rocks in, just rinse them thoroughly and possibly soak in clean water for a day or 2 (this is probally the better solution for removing bacteria and whatevers on your rocks)
NEVER USE SOAP TO CLEAN A ROCK! (residue will be left and leach into your water, killing your livestock)
if you are using store bought aquarium rocks, id personally use some sort of cleaning method for these as well, ATLEAST do a thorough rinse.
The way a few large rocks piled together is simple - it creates holes small enough for fry to get in and out but not big enough for adult fish to be welcome
(hope someone finds this somewhat useful