Joel, I'll take you at yo9ur word that the pond is 6000+ gallons. Let's go back to the comparison of a mud pond and a "natural" R&G style pond for a bit. The big difference with a mud pond (such as a Japanese breeders) is not only the duration and annual preperation that Russel so rightly pointed out, but it also has to do with the stocking densities. Stocking densities and how wastes are handled go hand in hand. In a mud pond, mother nature delas with such thorugh nitrification and EXTREME dilution which provides for extremely low ambient nutrient levels in the water. Even with this dilution rate and low stocking densities, the koi get and have parasites, parasite which must be treated for immediately upon removal from the mud pond every fall. Parasite thrive in nature but in our backyard recirculating systems must be reduced to levels such that our stocking densities and ambient nutrient levels do not cause parasites at epedemic proportions. Simpy put, R&G ponds cannot provide that environment over a period of time. With 1 mature koi in a pond suchg as yours, you have exceeded the stocking rates of a breeder's mud pond by 50 to 100X. With the lack of (or extremely low rate) of solids removal a R&G bottomed pond is doomed (sorry).
You didn't mention the turnover rate but let's do a little comparison here. Solids in a R&G pond will settle and fill the voids over time within the gravel base, left to deteriorate and adversely effect water quality at an increasing rate with time. A modern koi pond will sweep the bottom and remove those same solids to settlement for easy removal. Now, let's make an assumption here to make a point. Let's say we have 2 identical ponds, one without R&G and with a bottom drain and settlement system which is followed by bio filtration and the other with the typical "biofalls," skimmer (with minimal mechanical filtering capabilities) and an R&G bottom. Everyone got the picture(s) in their heads? Now, let's say that 60% of the solids fall from suspension to the floor of both ponds (a reasonable level, perhaps on the conservative side). In the R&G pond, these solids fill the gravel based voids and deteriorate over time adversely affecting water quality at an increasing rate. In a modern koi pond where the currents sweep the settled solids to the bottom drain, these solids are carried away to a settlement system to be easily flushed away (removed) from the system periodically (ussually once a week). Now, which provides for a better environment for water quality and thus our koi? I think that choice is obvious?
Now, let's go even a step further, shall we? Let's say that the settlement chamber is not dumped for 1 year. Now after a year, which pond has better water quality in the pond itself? The less knowledgeable may make the "leap" to say that both are equal? However, the solids in settlement that are deteriorating are still having the nutrient base handled by the bio filter immediately following the sump and thus the water quality in the modern koi pond is still better than the R&G pond that the same deterioration within the R&G bottom is directly and adversely affecting the pond's water quality and Koi's environment.
Keep in mind that is but an example and I would NEVER advocate not cleaning a settlement chamber for a year since the environemnt within it would still be highly conducive to parasitic and bacteia activity.
Joel, don't take any of the comment made (by myself or others) personally. Folks here are simply attempting to feed you some knowledge/ If your are happy with your R&G pond, hey....that is all that matters. But please do not promote such as a beneficial environment for koi (especially in comparison to better design methodologies.)...that is when folks can get a tad bit testi.
Edit: It would be interesting to see what the water volume is that is "trapped" within the gravel or an R&G pond. It would have to be metered until the point where the water rises above the R&G. This volume should then be removed from the total volume since it is uncirculated and eventuallt stagnent. Also, in the typical R&G style pond, the remaining volume is not "mixed" sufficiently to utilize our conventional thoughts on turnover either.
Steve