The gravel is mixed with black dry sand to help with filtration thought about using a UG on it but changed my mind the sand sifting snails due pretty well in the mix I original wanted banded pipefish with dwarf seahorses and an assortment of snails as CUC
(Used gravel sand mixed in my original 30 due to budget and kids wanted color wasn't up long enough for long term results but the time it was up it did very well)