MvM;584133 wrote: Don't know much about Zeo, but looking at the pic, I'm assuming that the cyano is feeding off of excess nutrients in the sand bed. IME, cyano only happens in new tanks OR when excess nutrients are introduced in overfeeding or in established tanks when die off (dead fish/inverts) occurs. That being said, I bet you have a pretty big build up of nutrients in your sand bed which is feeding off of the zeo you're dosing. This is the reason I prefer BB setups. The amount of nutrients that sand holds, whether thought to release into the water column or not, has to have an effect on water quality. Maybe way off base, JMO.