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when injecting co2 into the tank its normal for the pH to drop. 6.8 is fine.

It depends on your mixture, I use sugar, water and bread machine yeast and change it once a week regardless. I actually just "T" on another cannister and it dropped my pH to 6.0 wish is kinda low. You can use baking soda to raise the kh which will raise your pH. Its a process of finding out how much to add in your tank to keep the same reading. For example, 20 scoops the first time and after a 20% water change maybe 5 scoops? It varies from tank to tank.

Also, I see no other point in waiting out the entire co2 cannister life other than if you are extremely strapped for cash and cant afford extra sugar and yeast.
Keep that co2 pumpin in your tank, your plants love it.

 

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as long as co2 is being added to the tank the ph will drop and stay at a certain level. in your case 6.8 is fine and i wouldnt worry about it unless you have fish that absolutley cant live in it.

how big is the cannister you are using? Is it just the Haggen brand? How much sugar, water and yeast is added to the mixture? Like I said with mine it will probably last 10 days without any baking soda added. But I change it every week to insure I always have a good flow of co2 in the tank