Agreed, optimally the bigger the better, but there is a thread in the 'All things salty' forum that is saying even a 5 gallon bucket with DSB will bring nitrates down on a 75g tank. I have a small DSB in my fuge, and my nitrates are 0 (but since it was there since startup I do not know for a fact that it is due to the DSB, could be all the rock).