gravel? not much surface area for anaerobic bacteria to grow on with gravel, so no worries about anaerobic die off. with gravel or CC detritus collects and needs manuall removal (vac). now if you meant a deep sandbed (DSB) 3"-6+" depending on grain size, then yes die off will occur. with a DSB, if you only vac the top 1/2" it should be fine, dont go any deeper than that though. i would also only vac small sections off the top layer of the sandbed at a time. now if your talking about a short sandbed (under 3") then frequent vac maybe needed.Ok so does this mean you have a lot of anaerobic die off when you gravel vac?