they are relying on the phosphate sponge in the substrate. calcium carbonate is a fantastic phosphate binder. just like GFO and aluminum oxide. the more calcium carbonate the more binding sites available for the phosphates. lucky for us, the bacteria are able to remove phosphates from calcium carbonate. unfortunately there is gravity. over time phosphates migrate slowly down through the substrate until they reach the bottom of the tank. at which point the process stops, luckily for us i stops with the calcium carbonate having the last say since it is a true chemical binding reaction. the deeper and finer the calcium carbonate substrate the longer the phosphate sponge will act like a sponge until needing to be cleaned.
water changes do little for controlling nitrates and phosphates. water is just a carrier, it is not the source of the nutrients. that is detritus. if the detritus is able to work its way down through a substrate then there will be a low level of nutrients in the water column.
here is a chart showing the various nutrient export methods we use and which nutrients they go after. take special note of the arrow directions and the number of arrows going into and out of the substrate and algae (ATS's/Chaeto). note which methods make the most use of the arrows.
ATS do not remove phosphates from the system until they are exported. until that point the system is still becoming more eutrophic from feeding the system. looking at the chart above note that the phosphates that ATS/Chaeto (even GFO for that matter) get to only become available after the bacteria have released them from their organic sources. they are after the fact phosphate removers. a day late and a dollar short to the phosphate party. they are not helping to lower nutrient levels at the source. like turning on the fan in the bathroom to remove the stink instead of just flushing the toilet.
define 0 nitrates and phosphates. hobby test kits are not able to give us the precision we need to get to the zero level for algae growth.
phosphates on the reef are 0.005ppm, nitrates less than 0.1ppm. these would be zero levels for what we are talking about. unless we are reading to this accuracy, having anybody say they are reading zero is incorrect. the only indicator we have that our levels are here is if there is algae growing in the system. if non-hermatypic algae is growing in the system, then the levels are above oligotrophic level, the levels listed above.
any stirring or siphoning of a substrate allows new resources to enter the phosphate sponge and removes bacterial flock that can clog the sponge allowing the sponge to function properly. there is nothing wrong with using a substrate for what it does best bind phosphates, but in order for it to do its job it must be maintained just like any filter or sponge. it needs to be cleaned before it becomes so clogged that it can no longer bind any new phosphates.
from being on the forums for a while. the average time it seems for a substrate to start showing signs of eutrophication is one year for each inch of substrate in the display. obviously more phosphates sinks in other areas of the system can extend this, but this seems to be the formula. it is always a good idea to ask somebody who says they have zero phosphates/nitrates and does not do regular water changes how old the tank is and how deep is the substrate. it absolutely is possible and is shown all of the time, that this "works", but it will only "work" until the sponge is "full". at which point it needs to be cleaned. even in SW we need to clean up after our pets. we can not just hide it in the litter box forever.
G~
