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[APD] Low Oxygen levels

         I have been continuously observing CO2 & O2 levels in my 60 gal.
thickly overgrown planted tank with 3W per Gal. PLL lights. The normal
sunrise pH is about 6.6-6.7 and DO around 2 mg/L and at sunset the pH
around 6.8-7.0 range with DO greater than 8-9 mg/l (pearling)

         A couple of days ago I reduced the CO2 to observe the effect in
the time taken for O2 levels to saturate. I noticed a rise in pH as expected.
After 12 hrs the Dissolved Oxygen levels (DO) levels did not reach
saturation but was around 7 mg/l the next day at Sunrise the DO was
around 2 and pH 6.7. I increased the CO2 back to old levels.

         The 2nd day (yesterday) the pH levels were back to normal
6.6-6.8 range through the day and DO levels only reached a maximum
of 5.4 mg/l. Obviosly the effect of reducing the CO2 was seen a day later!

         This morning (3rd day) at sunrise, I found all fish at the surface
desperately gasping. The pH was 6.65 and the DO was just 0.6 mg/l.
I immediately added an aerator with a small airstone. The pH rose to
7.05 and DO to 6 mg/l in just two hours. Some fish have white spots
now, looks like the stress got to them.

         I should now switch over to a spray bar system without CO2
or find a way to increase the O2 levels without affecting the CO2..


  1. Add some aeration for a few hours at night with a timer?

  2. Fish load is moderate (recently reduced), trim back the plants ?

  3. Find a way to externally oxygenate the water without loosing much CO2.

         Any suggestion or ideas will be greatly appreciated.


Raj, vu2zap
Bangalore, South India.

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