I would go with the 50/50 rock idea. The bacteria will migrate to the base over time. The best reason for doing this is cost. You can't see what's on the bottom of the tank, so why spend $10 a pound for what you won't see. I don't know if you've priced LR but it can range from $5-$25 per pound depending on what kind you get. But really, I would suggest that if you really want to do a reef, research nanos, keep your big tank fish only until you are comfortable and set up a 10 gallon reef to start. It may be a bit tougher to set-up and keep thriving, but once you do get it going well, the knowledge that you will have gained will prepare you for anything you will encounter on a larger scale. Not only this, but it is much cheaper to light your tank as well, which is where you will spend the bulk of your setup money. You can take a 10 gallon tank use the standard hood and retrofit a 32 watt power compact fixture in it, cover tank with a glass canopy, filtration with a biowheel 125 (great for nano, not a large reef), add a 80 GPH powerhead and there is your setup. 20 times per hour turnover rate which is ideal for a reef, over 3 watts per gallon of lighting which many would say is barely adequate in a large reef (light intensity is the key in this situation though) and you are ready to keep anyhting you want, all for about $200 set up. Let it cycle with a couple of cheap damsels (very hardy) and you'll be ready for corals before you know it. You can add some LR at start up if you like, but it's not necessary. I would talk to your LFS and buy livesand (sand from an established reef tank) and some small LR pieces and go from there. You don't want to buy to much rock to start off cause you will be adding rock in with your corals. Before you know it, your tank will be full of rock with no room for corals. Buy a good test kit, follow the directions, buy some b-ionic calcium buffer system ($15 will last you forever in a 10 gallon), follow the directions, change water on a regular basis (10-20% weekly), do not overstock with fish, add a clean up crew (crabs, snails, etc.) and you are on your way to an awesome tank. Believe me, if you are setting this tank up for your mom to enjoy, you can't go wrong with a nano. You can pack a nano full with corals and such at a fraction of the cost of a large tank. Do a search of my username and you will see many progress photos from when I started my nano.