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KH drop means not enough CO2

Alex Pastor wrote:

>>The pH is 6.7 in the morning before the lights go on.  (3 watts/gal)
>>It is up to 7.5 by the evening when the lights go out.


>Carbonate hardness of the
>>water I put in is 3, but when I measure it in the tank at the end of the
>>it is maybe 1.5.  This concerns me. I assume it is because the nitrate is
>>not zero.

Russell Furlow wrote:

>First off sounds like you may not need CO2.

It sounds to me like he very much needs supplemental CO2.  The reason the
KH in the tank is dropping is because the plants are meeting their carbon
needs by splitting it out of the carbonate/bicarbonate in the water.
Without correction, this will lead to larger and more dangerous pH swings
as the buffering ability of the water is depleted.

>Second the pH flux is due to your plants and is normal. Remember CO2 to O2
>with light, O2 to CO2 without light.

I'm not sure what you mean by this.  But to clarify for those who are
wondering, plants respire (take in O2 and give off CO2) day and night.
During the day, given adequate amounts of light, they also use CO2 (carbon
is a MAJOR nutrient) and give off O2 as a byproduct of photosynthesis.  

>My DIY CO2 on a 55 gal planted tank with 160 watts of "cool white" bulbs, I
>know I've ordered 50/50's, has a pH "flux" from 6.8 in the morning to 7.2 by
>early night with 12 hrs of light.

Watch your KH.  If it remains stable, you are providing adequate CO2
supplementation.  If it starts to drop, you are not.