I've read that every 4-5 years a DSB needs to be changed out. I never read why tho (and don't really believe it). Damian, is this what you are talking about?
The theory is that sand beds and liverock act like nutrient sponges sequestering phosphate etc. as it is deposited into the substrate. At some point the "sponge" becomes saturated and then MIGHT begin leeching the sequestered nutrients back into the tank. How long does this take? No one really knows and I would suggest it depends a lot on the bio-load/ husbandry practices of each particular system.
The "sponge" aspect seems convincing enough. The leeching aspect is less convincing to me. FWIW, one of my systems has a sandbed that is approaching 15years of use. No problems in that system that I can attribute to the sandbed.
i took a powerhead yesterday and clean up all rocks and then took my net aand took out everything on my sand this morning my sand bed was getting covered with light spots of red slime
.....mine lasted about 7 years even with replacing sand..it got to a point that couldn't control the phosphates etc..as mentioned there are alot of contributing factors to it's life span. I went BB for the last 3-4 years and the phosphates dropped back down, but I missed the look of sand so I just recently put it back in...here is my thread from RC.. http://http://www.reefcentral.com/forums/showthread.php?s=&threadid=1549933
I normally do a weekly 10% W/C and run a phosphate reactor changed every 60 days. I decreased the bio-load,removing some fish etc..but still had to increase the phosphate media, replaced it every month and do bi-weekly W/Cs to try and keep water quality. It could have been a combination of old rocks and old sand because after I took out the sand and cleaned the rocks things were back to normal which is about .05 -.06 phosphates measured with a colorimeter....and more importantly cleaning algae off the glass only once a week...LOL