I don't think that's enough substrate nor the right type to be considered a DSB. [MENTION=3688]Illinijoe[/MENTION] - I'm curious which LFS gave you that advice.
To be clear I wasn't suggesting he had a deep sand bed I'm not sure how to even make one of those. My lfs recommended keeping my sand bed thin between 1/2 to 1 inch which is what I did. they said thicker ones trap waste
What's your thoughts?