Aragonite does slowly dissolve under aquarium conditions, but the process is slow and subject to a lot of variables. Many of these variables are a byproduct of confinement in an aquarium, which is why it's such a factor in a tank but not so much in the wild.
Other calcareous rocks also dissolve, but the oolitic size & shape of aragonite sand provides it with massive surface area exposed to water, thusly dissolving it faster. This is not the only reason aragonite is preferred over the other types. The shape of the calcium molecule found in aragonite makes it much more immediately bioavailable than other types of calcium.
The dissolving is of course why we even want the stuff in the first place. This is what makes aragonite so useful as a buffer and source of calcium/magnesium.
So why change some of it every six months?
Well, it's not really a matter of it being used up, but of being covered up... with bioslime. Over time big sections of your sandbed will become clogged and choked with various stuff which pretty much keeps it out of direct contact with the water in any meaningful way, and this in turn has an effect on the environment immediately surrounding the sand which allows for an undesirable trend of pH/alk decline among other things.
There are certainly ways around this, such as keeping the sand well-stirred or even using it in a reactor, but as already mentioned above, most people never really have any problems with this at all on account of making up for it with their other normal maintenance and dosing. Bob Fenner is simply telling you the perfect-world solution to the problem which will indeed give you superior results over not doing it. So, then, you can either not do it and do just fine, or do it and do even better. You can choose between higher maintenace but less sand changing, or lower maintenance and more sand changing. Sand changing gives you fresh minerals ( lots more elements locked up in aragonite than just calcium, you know...try about 80 of them ) coming at you at an increased rate, which gives your whole system a boost and saves you some bother with manually measuring and dosing all the time. ( again, a reactor helps a lot with this, but isn't crucial )
I've tried it both ways, and while I can say the sand-changing is worth the trouble for many applications, it isn't always worth it. I would do it if I were keeping a tank full of really demanding corals or if I needed to ensure a constant good crop of food organisms worth feeding to the fish, but otherwise I probably wouldn't. I'd tend toward and ecosystem filter in either case, by the way, but that's another thread.
Oh, as for innoculating your tank from an established one... that'll work okay if you absolutely have to, but there's hardly any real reason to have to. Just buy your sand & rock directly from the Florida rock & sand farms and have to shipped to you overnight. You'll get vastly superior product than anything you'll be apt to find in most petshops, and your tank will be instantly ready for fish the very next day, with bacteria, amphipods, snails.. the works. The price would be about the same, but the fresh stuff still has everything in it you want.
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