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There is some great information here! I have an additional question. My ph just before my lights and co2 come on is 7.6. My ph just before my lights and co2 go off is 6.8. MY KH is 4. I currently do not have fish in my tank, but hope to get them soon. My question is, is it OK that my ph is fluctuating this much every day???
Was looking for this exact same Q and A. Hope someone chimes in.
 

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You see a drop in ph because co2 forms carbonic acid with water, your actual ph never changes. If co2 actually changed the physical make up of water (GH and KH) then there would be no use for RO water systems. Co2 does not remove GH or KH, these are the factors that alter the real ph of your water.
This ph drop is a false ph drop. Think of it as co2 tricking your ph test kits or ph controller. Test kits and controllers are simply responding to contact of the co2 acid, the physical nature of the the water never changes, always remains the same. You can inject co2 until the ph controller reads 5.0 if you wanted to, the real ph is still the same. We know this because the minute you take a sample of the water in a cup, shake it up to gas off the co2 the ph go's right back up to where you started. Never once did you remove GH or KH from the water, the only real way to ever alter the ph is to use RO water, or rain water. These types of water contain no GH or KH, no minerals.
Outstanding, thank for the break down. I feel much more comfortable with the CO2 PH swings. Thanks everyone
 
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