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blue-green algae, Seachem test kit


I had an outbreak of blue/green algae (cynobacteria) in my 240g tank,and
tried adding various doses of nutrients to cure it.  No luck.  So I
changed some water, added a teaspoon of KNO3, and did a complete
blackout for 3 days.  Yesterday I took the blankets off, and the cyno
was gone.  I did a 50% water change.  I have a seachem Nitrite/Nitrate
test kit, and did the test.  The nitrite test was a bright
reddish/purplish color and showed a reading of 2.5. (This really
surprised me)  When I added the powder to test the Nitrate, it turned a
pale orangish color, which isn't on the scale (the scale has hues of
red).  If I tried to correlate the intensity of the orangish color with
the redness on the scale, I'd guess it was about 2.  But the orange
color of the water does not match up to anything on the scale (which has
shades of red).  So I added in 1.5 teaspoons of KNO3, K2SO4, epsom
salts, and trace elements.  Four hours later I was shocked to see the
cyno coming back, so I tested again.  This time the nitrite test was
very light red, and read .2 on the scale.  The nitrate was no longer
orange, but came up the proper reddish color and read .2 on the scale.
I also tested for sulfate and had .8.  I then added another 1.5 teaspoon
of the same stuff.  I just checked my tank this morning, and the cyno is
spreading.  So I did the test again and got 0 nitrite, and on the
nitrate test I got the very pale orangish color (I'd guess this was
about a 0 reading).  This puzzles me, so I did the test again an hour
later.  I got a reading of 0 nitrite (it was quite clear) and 0 nitrate
(completely clear--no reddish tint at all).  The variation between the
two tests is very puzzling, so I did the test for the 3d time this
morning.  I again got 0 for nitrite, and the nitrate was close to 0 with
a slight pale orangish/redish tint.  So there is some sort of
consistency in the tests.

A couple of questions:
Am I doing something wrong with my test kit when I get the orangish
color in the Nitrate test?  The scale to read the test has gradations of
red, and no pale orange.  The test kit is fairly new, and I am puzzled
by the orangish color in the nitrate test. 

Any other suggestions on how to get rid of the cyno?  I would like to
avoid using antibiotics (and I'd need a lot for a tank this size).

Should I be brave and add enough of the KNO3 until I read between 5 and
10 on my test kit?  This might take a lot of KNO3.
A couple of weeks ago I was adding up to 4 or 5 tablespoons of KNO3 a
week, in an attempt to get rid of the cyno.  The plants liked it, but
the cyno still spread.  Is it normal to have to add this much?

(tank info: 240g with a huge wet-dry filter.  light fish load (maybe 8
guppies, 12 nickle sized angel fish, 6 ottos, 8 SAE, a couple of corys,
4 small plecos.)  Over half the tank I have 4 T-12 40w tubes; the other
half has a 96w AH supply compact fluoro tube (the cyno is only on the side
of the tank with the 96w power compact light).  Plants (lots of val,
hygophila, ludwigia, swords, etc.) seem to grow fine, if slowly.)

As you can tell, I don't know a lot about this stuff, and any help would
be appreciated.  Thanks,