again, well said from somebody who has no clue on how phosphates and calcium carbonate interact.
first off, even the ocean ecosystem is not a closed system when it comes to phosphates. the may export method of phosphates from the ocean is plate tectonics. the abyss is a just a big ol phosphate mess. there is some recycling of phosphates through deep upwellings, but phosphates themselves are not exported from the ocean, yet there is a constant influx of phosphates from ground runoff. read up on the phosphate cycle on earth to get a better idea on this. the point is even if we were to "replicate the ocean" it would be incomplete. we would not be exporting phosphate, we would still be creating a system that is becoming more eutrophic with every feeding of the tank. in order to have a system that lasts indefinitely one must have a mechanism for inorganic nutrient export. a DSB system lacks this. what makes a DSB system "work" is the calcium carbonate abilities to bind inorganic phosphates, not the other organisms in the substrate. for every organism in a substrate represents an increase in nutrients. you can not have more organisms without more food to support them. the more food, therefore the more nutrients there must be available in the system. you can not have one without the other.
back to undergravel filters, since you mentioned them. do i believe in them, nope, but i think that they are a better system, then a DSB. a DSB system as mentioned before does not have an inorganic nutrient export method that goes after these nutrients within the substrate, before you say algae, read up on the differences between true plants and algae and holdfasts and roots. an undergravel filter at least tries to take into account the fact that bacterial flock will accumulate in a substrate and tries to create a way to get to it in order to export it. does it work, not all that well, but the thinking is more sound.
DSB's can work for certain biotopes almost indefinitely. the problem is that it does not work for most of the biotopes we want to replicate in our system. if what you want is a biotope that is more eutrophic, meaning it needs a constant supply of inorganic nutrients, then the DSB system is fine choice, with very little maintenance needed here and there and just minor substrate replacement to maintain the desired levels of inorganic nutrients. if an oligotrophic biotope is what you are after, then the DSB is not a wise choice long term. it will work ok, until the calcium carbonate has absorbed all the inorganic phosphates, then there will slow climb in phosphates levels, which will become unhealthy for the oligotrophic organisms wishing to be kept.
people seem to blindly recommend the same type of setup to all, i think this is a problem in our hobby. the oceans of the world are filled with countless different niche environments. to think that we should use the same setup for all of these is just silly. these different environments have evolved for a reason, we need to understand these reasons and create the system to match these environmental factors in order to have a successful system for those organisms from that environment.
G~
