What brand is it?
I had the same trouble with Marina black coated gravel.
After a few days I took it out when I narrowed it down to the gravel ruining the water....it almost killed my fish!
I kept the gravel tho....I plan on using it as decor later.
I switched later to natual gravel that I got a my LFS.....if you get the natural stuff make sure you rinse it extremely well, there's a lot of dust, sand, and dirt in it!