There are ssssoooooooo many variables that I cannot answer that for you. Eventually, a DSB will fill up (anaerobic zone gets bigger while the aerobic zone gets smaller). When that happens, you will get the hardening that Tangster is describing. However, it's been my experience that most people upgrade their tanks before this happens. If you are doing the right thing for your sandbed (see several posts up), you can have one last years. This is very true and is responsible for short-lived DSB's. Here's Dr. Ron's comments on how much bioload a DSB can handle. Most people I know that have a 100 gallon tank have waaaay more than 3 or 4 small fish and a couple of corals. Any more bioload than that and even the DSB's staunchiest proponent admits that it's too much. Here's a thread you might like. http://www.3reef.com/forums/sand/bb-vs-dsb-34593.html?highlight=bare bottom DISCLAIMER: I'M NOT A FAN OF A LOT OF DR. SHIMEK'S SLOPPY WORK AND OCCASIONALLY I WILL MAKE FUN OF HIM IN PUBLIC. HOWEVER, THESE ARE MY OPINIONS AND NOT THE OPINIONS OF 3REEF.