Are you suggesting that a DSB by itself is adequate bio filtration to handle all of the heavy feedings that are required to maintain 5 broodstock pairs? How much flow does this DSB need? My fuge has very low flow. I'm not convinced that a DSB by itself can convert all that food and waste to nitrate.
Nope, that is not what a non-critter based DSB does. Not entirely anyway—but neither will bio-balls. In the upper oxygenated layer, which may only be a cm deep if that, there will be processing of of dissolved organics -> Ammonia -> Nitrite -> Nitrate, but this is a process that occurs on every surface, substrate, pipes, inside pump volutes, walls of the tank, the surface of rock, and other shallow sand beds. The beginning of the process is accomplished by heterotrophic bacteria, that decompose the organic matter, and dissolved organics to Ammonia. This process also occurs in all parts of the system. What the non-critter based DSB cannot deal with is the particulate matter, and neither can the bio-balls. Such particulate matter is not much of a problem as long as it is not collected, or allowed to settle out, where it will immediately begin to decompose adding more dissolved organics to the system.
There is a difference between Marine systems and Freshwater systems. Marine fish, do not excrete ammonia, as fresh water fish do. They excrete dissolved organics directly to the water which, along with excess food, must then first be converted to ammonia, before going any further. Although the conversion process from ammonia to nitrate is very efficient due to the large populations of autotrophic bacteria in a well managed system (initial cycle,) the decomposition process is slower, because the heterotrophic bacteria populations are much lower, as they must be for the system to remain in balance, and capable of recovering from ammonia spikes quickly without losses.
The result is a steady build up of dissolved organics, that pollute the system, and can easily cause a systems demise over time. This makes the basic problem with marine systems the dissolved organics, not the ammonia -> nitrate process. Bio-balls address the Ammonia -> Nitrate process, as well as heterotrophic processes, but do not turbo charge anything enough to make them worth anything. In terms of populations, there is more surface area for colonization of autotrophic bacteria on a couple of rocks, and a couple inches of sand than there is with a bucket of bio-balls, so really what is the point? The process is every bit as efficient when the substrate is submerged, as it is when exposed to air...hence the bio-balls do nothing but produce nitrates, which is hardly if ever a problem in most any system, the problem displays as the all too familiar nuisance algae problems due to high nitrate production.
There is only one method to directly remove the dissolved organics, and keep them 'in check' at least, so they don't excessively build up. That is a large powerful skimmer, usually three times larger than what is recommended by the manufacturer for a given tank size. At that, you have a skimmer that is sized properly for the system. (There are no standards for skimmer sizing, it is all marketing hype.)
Armed with a properly sized skimmer, and we already know the ammonia -> nitrate process will produce excess nitrates, without adding bio-balls, the only thing left to deal with is the removal of nitrates more efficiently than water changes can even come close to, and that is what a DSB does. Denitrification. Bio-balls don't do it, and neither does rock (there is no mechanism for water movement through the dead base rock that is called live rock these days.)
You say nitrates are not an issue with the system you intend. That is great, but it actually is....if it gets out of hand, and out of hand it will get, unless you deal with it.
The flow rates for a DSB should be in the so called "SPS flow rate" range (such as 200 gph over the surface of 60lbs of sand in a 5 gallon bucket, which results in only a few inches of water above the DSB.) Low flow through a sump with a sand bed in it is one of the worst practices around.
If you were planning a 100 or a 1000 breeding tanks, then maybe some thought should be given to the methods used, but on a tiny scale there is not really much point, in the end what you want is excellent water quality, and the best water quality is in well managed reef systems. I have 25 breeding pairs, various species, and they all inhabit "reef environments." I have breeding pairs in my main display tank...a bit more dangerous for the offspring in there however...
The real question is: Do you want to produce nitrates, bio-balls do that very well. Or, do you want to produce excellent water quality, which is what your breeding pairs want and need, along with some other things....low to zero nitrates is a part of high water quality...nitrate is a pollutant...though not toxic at levels normally found in aquaria, and neither is nitrite...in that sense your thinking is correct.
Well that is how several other folks see it and practice it....
