Diatoms don't grow on the glass. I would be more inclined to say you have cyano
in the system, but would need a picture to say what is really going on.
Diatoms will die out on their own once the silicates in the system are consumed. It is hard to believe this is still on going 3 months down the line.
Adding livestock is never a solution to one's issue as it only adds more eating/pooping creatures to the system that is already telling you there is an issue going on.