A great example of how opinions can vary greatly in the hobby. My view on the subject getting a little deeper than just saying "don't disturb your sand bed": - Sand beds harbour both aerobic and anaerobic bacteria. By definition, aerobic bacteria need oxygen to live and survive. Anaerobic do not. But it goes further than this. Anaerobic bacteria either can't tolerate normal oxygen levels or get beaten out by aerobic bacteria in aerobic zones. If they could then we'd see anaerobic bacteria all over the surfaces of aquarium "stuff" and on the sandbed. We don't. - When you shove a vacuum into your sand bed you the sand harbouring the aerobic bacteria mixes with the sand harbouring the anaerobic bacteria and some of the former ends up on the bottom of the sand bed and some of the latter on the top. This means that this portion of aerobic bacteria ends up in an anaerobic zone. It needs oxygen, doesn't have it, and it dies. This portion of anaerobic bacteria can't tolerate oxygen or gets beat out by aerobic bacteria and it dies. So by putting that vacuum in your sand bed you've just killed a portion of your bacteria colonies by putting that portion in a location where it cannot survive. - If you're going strictly for looks and have only a shallow sand bed then to my knowledge vacuuming wouldn't affect you as much since you would be only hitting the aerobic "layer" (since there isn't an anaerobic layer). You'd lose some bacteria out the vacuum. - There is debate that when you vacuum you remove a hefty portion of the bacteria along with the water going through the vac since the sand swirls around in the tube for a while, giving the bacteria a chance to dislodge from the sand. I'm not sure about that debate, but I certainly do believe that not all bacteria is attached to sand and a lot of it goes down the drain with the vacuumed water.