I have this weird black tar on my HC Cuba and UG, it kind of resembles a black smug on the plants.
EI dose [NPK dry dose] + Flourish + Flourish Iron
25% twice weekly water changes
42 LEDs 7000k (This is not very bright, don't be fool by the 42)
CFL 6500k Desktop Lamp
Photo Period: 10 hours
Co2 pressured : 1 bps