Update 2: I removed all the white gravel and put in the diamond black sand.
The white gravel experience is not pleasant. It's fine gravel by the way, perhaps as much as 1.5mm per grain. It traps a lot of dirt, dirt is easily visible and then my new stingray is not able to lift it at all. What made is worse is that I added pebbles into my tank. The pebbles are supposed to cover the aerators. In the end, dirt is trapped in between the pebbles at the back of the tank. And because it is a cichlid tank, most of the pebbles are scattered by the 2nd day I bought them.
The new black sand is about 0.5mm per grain. Very heavy and is not easily moved by moving fish but light enough for the ray to bury under it. The ray is pretty active now and seems to enjoy the sand a lot. The other fish seem to like black sand as well. The tank however is too dark at night and I cannot even see where my fishes are specially my nocturnal clown knife. Best thing is the very dark sand brings out the colors of the fish and it is like putting them in the foreground. Pictures to follow later when optimum visibility is reached.