People on a couple other forums keep insisting that a sand & gravel filter is not a good biofilter, yet here on Koiphen, the opposite statement goes unchallenged. The main argument against the SG as biofilter seems to be that when the filter is blown out, the bacteria will be stripped off the media. Is there anything to this idea? Or is there some other reason why an SG filter wouldn't make a good biofilter?