Curtswearing
Mantisfreak
When you say crusted, does that mean that the sand particles are cemented together or is the grey layer detritus or detrital mulm?
You should test for all of the normal things. Ca, alkalinity, Mg, A, NO2, NO3, phosphates. Depending on how much cyanobacteria you have, you might not get a reading for Nitrates or phosphates because the cyanobacteria is likely absorbing it out of the water column too fast for you to get a reading.
You mentioned on one post that you had a brown algae and now you're discussing red cyanobacteria. Do you still have both. The reason I'm asking is because dinoflagellates love high flow whereas most cyanobacteria doesn't.
Neither cyanobacteria nor dinoflagellates need much in the way of lighting to grow. They can fix their own nitrogen right from the Ammonia, Nitrites, and Nitrates in your tank. Additionally, most tanks have more than enough phosphates to feed them. How do you limit your phosphate inputs and how do you export your phosphates? As mentioned previously, siphoning off cyanobacteria or dinoflagellates is probably the best phosphate exporter there is. No macroalgae even comes remotely close.
