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[APD] CO2: How Much Is Too Much?

I recently replaced my 30-gallon tank with a 50-gallon tank. When the initial bits of algae had disappeared, I decided it looked good enough to take some new pictures for my web site. I didn't like the looks of the bright yellow pH probe sticking down into the tank, so I pulled it up out of the way. Most of the pictures turned out O.K., and you can see them if you wish at www.fitchfamily.com/aquarium.html.

BUT, when I was done taking pictures, I forgot (well, I'm 79) to push the probe back down into the tank. And so to bed. When I sat down to breakfast the next morning, I noticed that some of the fish were up at the surface. Then I realized the pH probe tip was probably completely out of the water and measuring the pH of who knows what. I looked at the controller and saw that the pH had dropped overnight from 6.7 down to 5.9! So, all night the probe was telling the controller that the pH was above 6.7 and to keep pumping in CO2.

A pH value of 5.9 is off the chart and the table I have, so I used the formula:
CO2 = 3.0 * KH * 10^(7.00-pH). If I did the math correctly, that means that, with a KH of 5, the CO2 concentration was about 189 ppm! I think I've read that anything over 40 ppm can be lethal to fish. As fast as I could, I siphoned out water at one end and pumped in water at the other. It took hours to get the pH back up where it belonged. Meanwhile, some of the fish were doing barrel rolls and otherwise behaving rather poorly.

Well, the upshot was that I lost four Neon and/or Cardinal Tetras and one Harlequin Rasbora. But, to my amazement, the other 15 or so Neons and Cardinals, the one other Rasbora, the Clown Pleco, the Albino Cory, and many Amano and Cherry Shrimp seemed completely unaffected -- not even swimming up the surface. And, now, a week later, they're still apparently as healthy as ever.

John T. Fitch
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