My derasa is out growing the crocea two to one, but then, derasas always out grow croceas two to one. To me, if it was water chemistry, the derasa would show some sign of a problem since he's growing faster and has a higher intake of elements in the water, and he's normal. And the crocea is more than just a round shell, it has the longest mantle extension of any clam I've ever seen when it's wide open, almost leafy. It's not starved for light, it's straight down, 14" under water from a 250HQI, 4" off the surface.