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Green Water

I have been battling a case of green water now for about a month in a newly
established tank.  If I don't do anything about it, visibility goes down to
about 3 inches.  In this state the plants are not receiving enough light and
can't grow to compete with the algae.  I tried changing the water, against
the recommendations I've read here, and the phosphates in the tap water
cause the algae to return full force in about 4 days.  I tried my diatom
filter that cleared the water up to 50 % visibility, but the green returned
soon after I removed the filter.  I don't want to wait it out as my plants
are slowly dying from lack of light.  The fish are doing quite well.

The parameters of the aquarium are.

75 gallons
PH 7.0 directly from the tap.  Goes to PH 8.2 without CO2 injection.  When
the water is really green the PH creeps up to about 7.4.  This is with about
4 bubbles per second CO2.
KH=220 ppm
GH=110 ppm (?)
Phosphate=.5 ppm
ammonia, nitrite, nitrate = 0, but I have tried 15 ml TMG with 1.5 gm K2NO3,
same results.
lights are currently two 175 MH pendants.
temp night = 75 deg f, end of light period 78 deg f.  duration = 10 hours.

I just read that someone was successful using a diatom filter.  I have an
XL.  After about a day, the diatom mixture was pale green, but the water
would not clear past 50%.  I also read in Innes that potassium permanganate
will clear green water.  I don't want to hurt the plants, nor fish.

I was thinking of using an additive to get the PH down into the low sixes.
I think the CO2 injection can't help below 7.0 because of my water hardness.

There is no surface algae anywhere in the tank thanks to 2 SAEs, 4 CAEs, and
2 small plecos.

Any advice would be appreciated.  I just want to get over this so I can grow


Steve Pituch
Katy, TX