sorry to hijack your thread .. but i juz wanna ask, my sand bed is only about 2 maybe 3 inches at the deepest point however i got little bubbles forming. i am using grade 0 sand (very fine) the bubbles are produced all over in the sand bed, i know this because i every stirr the sand before in the past. could it be nitrogen gas ? bubbles only appear ard 2 cm below the surface of the sand