What kind of flow do you have, what does your clean up crew consist of and do you ever stir the sand?
You need lots of flow, a good crew and never stir the sand up or you upset the bacteria growth and run the risk of it going septic and killing your tank.
Thats not a tremendous amount of flow. A SCWD will cut the flow by 25 to 30% so you are probably closer to 500 GPH. Thats about 12x turnover for a 40G tank. I am at 36x turnover in my 100G and looking for more.