There could be a few reasons for this, but i would guess there not happy. A fish that is getting sick will loose its eye color sometimes before any other symptoms show up. I could be wrong here, but it is my guess.
Blue diamonds and other solid types of fish will sometimes have there eyes change from red to a yellowish color, but i have never seen it in any turk based fish without an underliing condition.
I would at the very least up the water changes to every day at the minimum of 50%.
HTH, Matt