I have a 75 gallon planted tank, CO2 system, adequate filtration, not too many fish, correct lighting, temp maintained at 78 degrees F. I feed the plants on alternate days with Flourish Excel in addition to the CO2 infusion. Tank does not receive direct sunlight. Green algae present but not a problem. The problem is black crusty formations on many of the plant leaves. I have determined that this may be diatoms but may be brown algae.Water chemistry is within normal parameters (ph, ammonia, nitrate, nitrite). Water changes at 40% replacement every three weeks, distilled water or rainwater used. Does anyone recognize these symptoms and if so, how do I get rid of them?