its dark grey the pics bad but its i think hagens coated gravel i didnt want any extra bedding for bacteria my wet dry already produces too much nitrate if you mean substrate under the gravel none i'm adding flourish for carbon and iron food spikes
better look that one up peacock nitrates are a food source for plants the most effective way of keeping nitrates down is planted tanks my main aquascaped tank has not had any nitrates NO3 since i added them back when i couldnt control it with water changes anymore