with cyno there,s several reasons , 1 exactly what frankie referd to, not enough flow stagnet area,break down of ditrius bacteria thru the sand bed, now with a combo of tap water not enough flow and not stirring the sand bed reguarly, can cause this. the bacteria can be frustrating to get rid of. there are ways to get rid of it with out chemicals, water changes with ro water some gravel vaccing and changing flow, you can also get a tiger cowery snail and they will mow thru it. i find that by having a preaty good load of sand snails will keep it stirred up.now cyno can spread to corals some corals can die from suffacation, but i have noticed from experimenting that some corals can get some nourishment from it. thanks fish