Since the temperature of the solution went up over 70 degrees there has again been noticeable growth, this time, with the spinach. The burst in growth, I think, was probably caused by a combination of the warm solution, a new clay growing media, and extra time to adapt to that media.

As you can see, I've thinned out most of the withering leaves. Hopefully the energy being sapped by the dying leaves is redirected towards more leaves.