My 110 gallon tank has been running for a little over 3 months. I am going through the ugly stage and have diatoms all over my sand and rock, however, today I came across a purple spot on my sand. Nitrites 0, nitrates are between 0 and 5 ppm, ammonia 0, PH 8. I've heard that this could be cyano, but maybe it's what comes after diatoms? Is there something I should do or just let it be? Thank you.