persistent algae

["Roxanne Bittman" <rbittman at dfg_ca.gov>]

I still have an algae problem in my 40 gal tank.  It's mainly on the 2
large pieces of wood and it is primarily red (the "black brush" type).  
There is a small amount of green on leaf tips (hair-like); no glass
algae and no Cladophora on gravel (yet).

Parameters:
40 gal
high light (2x96watt power compacts), high CO2 via pressurized gas and
internal reactor
KH = 4, GH=6
pH = 6.5
NO3 = unknown, 'cause I stopped looking.  It was between 1-10ppm 2
weeks ago
PO4 = 0.5ppm
iron = 0.1ppm
K = higher than my Aquarium Landscapes test will discern , >2ppm

My goal is no visible algae and the tank has been set up since Nov 2001
and still no success.  I have tried the method of feeding the plants so
they grow so well they outcompete the algae and that resulted in two
things: GW (mild) and hair algae (heavy).  Not good.  I cut back on
fertilizing everything but traces and the greens receded.  The black
brush algae infesting the wood is the current winner and it's really
ugly in my opinion.  I've never had this, in over 10 years of planted
aquarium keeping.

My current suspicion is that this is the result of persistent high K. 
I know, you can all start throwing rotten fruit anytime...but, that's my
current theory.  K was well over 20ppm.  To see if the high K was the
problem (by any chance) I decided to test for it and bring it down. 
This process has been taking many water changes.  I have made 50% water
changes once/week for 3 weeks and the K still is over 2ppm (the maximum
amount detectable by the test I have).  I also started adding barley
straw extract, so I changed two things (at least) so this is not
science, too bad.

The main change has been that the black brush algae turned light green.
 Yes, it's not black anymore.  However, it seems fine in other respects
and still appears to spread.  Unfortunately, I have noticed no algae
death yet.

So, how are the higher plants doing you ask?  Absolutely fine.  I
change 50% water as described, and only add back traces (TMG) at the
recommended amount (not more, as I used to).  I add no KNO3, and no
K2SO4.  When PO4 hits zero, I add a tiny spoonful (spoon from a test
kit). No plants look stressed and they still need trimming weekly.

So, what do you all think?  Will reducing the K concentration help; has
anyone else tried this route?  Anyone else had success/failure using
barley straw extract?  What does it mean if the algae changes color?  Is
2ppm K "way too low" in your collective opinions?  I just know that, in
all the years of planted tanks, this is the first time I have any
problem with red algae.  It's either due to the Onyx or the high K (or
the higher light) since those are the things that are different.
I also wonder if the wood is taking up too much real estate; it
probably covers 1/3 of the tank bottom area.  Ideally, it would be
better to have fast growers there, but I like the mossy wood...

Thanks for any comments.

Roxanne Bittman