i would check then. if it is not able to read with 0.001ppm resolution, than it is not adequate. the last time i checked. the Salifert phosphate kit can only read with 0.01ppm resolution. which is not going to be adequate.
how long it takes for a DSB/RDSB to "fill" up depends on a lot of factors. i talked about it a little earlier in this thread, but we can go over it again if need be. the speed of the flow over the RDSB is double edged sword. the enemy here is the detritus, critter poo, and bacterial flock. low flow allows detritus to settle. the question is, where do you want it to settle for easy removal. if the RDSB is the best place someone has for collecting detritus not in the display, then i suggest a slower flow and manual removal of detritus once a week during water changes. if there is a sump or conical settling tank that collects the detritus, then keeping good flow over the RDSB would be preferred. the detritus is the enemy.
as for PO4 in the water column. how does it get there in the first place? if all we put in the tank is organic phosphates in the form of food, then how do we get inorganic phosphates in the water column?
confused on the formation of calcium carbonate comment. people still believe that the substrate acts as a buffer for Ca and alk?
i thought this was disproven many years ago. it is when the pH becomes low enough to cause the dissolution of calcium carbonate in the substrate that H2S is allowed to start forming under these dissolved areas that then coagulate when pH rises above the dissolution pH. what happens when a calcium carbonate that is loaded with phosphates dissolves? where does the phosphates go? the same goes with a calcium reactor. they add phosphates to the system. why would the use of muriatic acid to "remove" phosphates from LR not also release phosphates in a calcium reactor? what is the difference? calcium carbonate is a phosphate binder. it wants phosphates. dissolving phosphates just releases any bound phosphates back into solution along with the calcium carbonate.
the point of any calcium carbonate in a system (besides the skeletons formed by hermatypic organisms) is to bind phosphates. we need to treat those calcium carbonate media that is not self cleaning (substrates, LR is able to self clean because of flow flow all around it to remove detritus), as the filter it is. it is only able to bind so much inorganic PO4 by itself, and the critters that are involved in the substrate are only able to bind so much inorganic PO4 also. at some point the amount of critters and detritus reaches a tipping point and the entire substrate (critters and calcium carbonate) can not take anymore and the phosphate are no longer able to slowly migrate downward through it.
the good news is that refreshing a substrate is easy. it only takes a good stirring and rinsing (preferably with SW if the media wants to be reused). this is very easy on a RDSB. just get all of the detritus out of the entity on a regular basis. this could take years, or just doing it a couple of times a year is sufficient. if you feel that it needs to happen "just like nature", then think of it as a tropical storm or any large storm that goes through the tropics. it stirs up the substrates. allows accumulated detritus to get washed out tot the abyss, and allows the layers of calcium carbonate to start the migration of phosphates back down through it again before getting clogged by detritus.
back to the comment earlier about the formation of any of the nitrogenous compounds by bacteria. it is like saying all you need to do to create O2 from CO2 is add a plant. taken at its simplest level, this is correct, but you also need to include all of the resources needed to support the plant to actually get the O2. you need the sunlight for energy, you need the phosphates and nitrates for the formation of the plant itself. the same thing goes for bacteria. the bacteria also need resources in order to live. it is these resources that must also be considered when trying to figure out what is going on in our systems. it is super easy to just ignore phosphates, this has been especially true when the levels actually needed to limiting to algae growth are so low.
G~
we are getting closer and closer to getting test kits precise enough. we have just been told the wrong way to use algae. 
i bet it reads 0. 
if your test kit were to read 0 phosphates, and your nitrate test kit read 10, which nutrient would you blame? for comparison the oligotrophic environment level for nitrate is 0.9ppm. does your nitrate kit read to that resolution? 
