We used STG (or SureGrow), the loose-fill like it looks like you have, and we begrudgingly went back to rocks because the STG holds too much water (we still use the STG 'cubes, surrounded by rocks in the net pots). In DWC the STG loose-fill was an issue for us, and I hate stepping on those damn rocks!

I'd agree with some of the previous posts - your water level is too low, the water level needs to be about 1/4-1/2 inch above the lowest roots (at a minimum), at least that's my 2 cents.