Cyano is the simplest most persistant form of life, and once its inroduced, and the conditions are right, it can be very difficult to get rid of. Its normal for it to die back at night, its like diatoms which will grow rapidly when the lights are on, and die back when they are off. I would do several water changes in a row, removing only enough water to allow you to syphon out all the cyano each time. It grows best in areas of low current, those power heads may help out. Fresh activated carbon may also help. You might also try some more red legged hermits that will dig through it looking for goodies in the sand. HTH