Could be a number of things. With plain gravel and no ferts in the water column, my first guess is a lack of iron and/or other nutrients. I'd try using a trace mix with iron and see how they do.
Thats what I heard too but Doctors Foster and Smith said 2 WPG would be enough. Anyways I am planning on redoing my substate and going with flourite and using flourish as a fert so hopefully this helps