Black slim

Question
QUESTION: Hello my 14 gallon tank has been running for 6 months now and recently a black slim has appeared do you know what it is . Thanks

ANSWER: Is this fresh or saltwater?

---------- FOLLOW-UP ----------

QUESTION: Saltwater tank

Answer
It's not black slime, Ben.  It's Cyanobacteria.  In the light, it's actually going to turn maroon...if lighting is on.

It comes from overfeeding, tap water, and improper filtration.

Do water changes using R/O water.

It's the best cure.

Happy fish-keeping.