just thought id share an interesting conversation from another forum about dsb's having to be replaced completely due to their nature of absorbing nutrients and gaseous nature. Here's a quote.
It was my understanding that in a reef tank we are simulating the effects of a natural ocean but as our sandbeds remain static and not moved around by tides it would seem to me that this quote is true and that indeed the buildup of waste in a dsb is an inevitable fact.
DSB's are nothing more than a nutrient sponge in your tank. Period. You can try to explain it away as much as you want, but in the end their sole purpose is to soak up the built up nutrients and junk in your tank. The problem is that we're dealing with a relatively small and finite volume...and it's only a matter of time before it gets full. That's when stuff hits the fan. If you visit various other reef boards, you will see a rather common occurance of those who have had their DSB's up for 4-5 years...and their tanks have gone straight to hell. Even the LR becomes oversaturated with nutrients, leading to uncrontrolled nuisance algae, coral bleaching, etc. in tanks that were otherwise thriving and healthy. Once the DSB is saturated, the only option is to COMPLETELY REPLACE IT. You can spend money every year introduce new fauna packages from various vendors and recharging it with biodiversity on an extremely regular basis...but at some point its going to reach maximum overload. All this to avoid siphoning out some crap every once in a while.
It was my understanding that in a reef tank we are simulating the effects of a natural ocean but as our sandbeds remain static and not moved around by tides it would seem to me that this quote is true and that indeed the buildup of waste in a dsb is an inevitable fact.
The problem is that the conditions of a typical DSB is the complete OPPOSITE of conditions on the natural reef. In nature, much of teh sandbed is actually aerobic, with smaller portions being anaerobic or anoxic. In our systems, it's the anoxic areas that comprise most of the DSB because that's the part that takes care of denitrification and that's the process we want to occur. Not so on the reef. The DSB is NOT creating natural conditions in your tank...don't be fooled by that statement.