An epoxy putty made for marine aquariums would work. You just mix the two sticks together, shove it in the cracks, and then push the gravel pieces into that. You have to work kind of quickly. A more liquid epoxy may also work, but I don't know if it's safeness to aquatic life.
A clear aquarium grade silicone gel would work as well, and you'd likely get very good coverage with it.
USDA Zone 7b/ Sunset Zone 31