tripdad
Frustrated Stick Gardener
I will have to look for it but I remember an article I read that when nutients in the form of nitrates and phosphates are driven low to zero that bad algae's can "change" there food source and become bacteria feeders instead. This would jive with what your getting. If nutrients in traditional form are zero'd then corals will starve and if algae is consuming bacteria then your tank will become even harder to get a true balance in as it needs bacteria to run the cycle. I may be remembering wrong but this is what I remeber. I will look for the article. I would think that any detectable phos or nitrate would be enough but who knows for sure, kinda "black art" stuff coloring these things up. Best advice so far is go slow with any changes.