Hmmm ok, next idea. I remember reading in Trop Fish Hobbyist way back in the late 90s maybe of someone spawning uaru amphiacanthoides in a tank with all sides completely blacked out except for a small viewing window in the front. You could try covering the back and sides of the tank to give them more security & help them settle in more - not sure if you've done this already, just grasping at straws here.