If the black stuff is stuck to the plants, you likely have BBA rather than diatoms. Diatoms comes off very easily and looks brownish, and is like dust. BBA looks like tuffs of short black threads <hence black brush algae> and sticks like glue to everything.
I have been battling BBA for months. In a non-planted tank, you can get rid of it with a total blackout but it will take days rather than the 24-48 Hr needed for green algae (someone here had to do it for 2 weeks). If you have BBA, upping the light might make things worse.
BBA is more common in cold water (like a goldies tank). I have noticed that stuff I moved over to the 5 gal (at 80F) have the BBA disappear after a week or so. I haven't tried it in the goldies tank, but cranking up the temp may help.
The root cause is some kind of nutrient imbalance. With goldies being the pigs they are, the tank is prob high in NO3, PO4 & other algae creating stuff. Major water change & gravel vacs can help, but with goldies, you are looking at something like 70% changes every few days to keep the levels down. In a planted tank, adding fast growing plants is one way to soak up the nutrients. I have added hornwort (a single bunch that grew to about 1/3 of a tank full in a couple weeks), and some SAE's, and things seems to be finally heading in the right direction.
