Would it work to let the concrete cure for a full 30 days before submerging it? By then the cure should be complete and there should be nothing left to leach out. I would think that the only thing you would need to do is rinse off the outside. Just a thought.
That makes perfect sense, since you have all the concrete research ever done and experiences of professionals backing you up. However hobbyists are impatient, sure they can find a shortcut, and sure the rules must be different for them since "after all they are not making a sidewalk". Most of the curing will be done after the 30 days of misting or loosely wrapping it in plastic, however the curing of concrete keeps going on, ever more slowly. There probably is no reason to do water changes, or to drop the rocks into a running stream from a hose. You are not really leaching out alkalies in significant amounts, you are creating a biological film that seals the rock from the water, the same biological film that processes ammonia and nitrogenous wastes from fish in a biological filter. It takes about two weeks for the "cycle", and no surprise, the same amount of time for the rock to "leach" out.
My method is to set the rocks, driftwood base, cave, or background into a large container, fill it with water and some oak leaves, add several Gambusia since they tolerate high pH, the nitrogen cycle, temperature swings, and as a plus they devour mosquito larvae if you do this outside. If it rains, there is a "water change". If not, there isn't.
And PS, there are some shortcuts. It's just they are not as cheap as Portland cement, and require techniques that are harder tro learn, and less forgiving of mistakes.