BGA in 3' tank

Ah geez, I've neglected fertilizing my plants in my 3' tank for the past two
months and after 3 years now have what I hope is only BGA, well its very
black and slimey . I cured it in my 4'er when I first set it up just over a
year ago with some great advice from Chucky and haven't had it since since
the plants have really filled in. Looks like a 3 day blackout is on the
books. I've only got hygro difformis which is growing almost emersed size
leaves underwater, blue stricta which recently frizzled just at the top and
an amazon sword which is a tad pale in the 3' and after testing the water
I've got no nitrates, .8ppm phosphate and probably no iron.