<shrug> I've seen FW Tanks in the high 5s. I would assume with no carbonate solids in the water dissolving (ie. no substrate, no calcerous rock of any description) it would behave much the same way for salt. It's not very bloody likely to happen, but I'd be willing to put a small bet that if you hooked up a HOB filter to a small tank of saltwater and just slowly fed it ammonia to see how far the bacteria could take it, that it would drop rather horrifyingly low. (Bad, bad, bad sentence) Alas I lack the dedication to run such an experiment for the weeks it would take. But I don't claim any reef tank can ever get that low - only that a tank with 0dKH can rocket down there pretty sharply.
Rock most certainly buffers the water.. With a ton of live rock and a sand bed, admittedly, still not enough to compensate for what corals suck out, but it's definitely happening.
An intellectual exercise; certainly not a realistic illustration.
