I set up one bucket with my drilled rubber ring hung and another bucket with a large, cylindrical air stone. I recalibrateted my dissolved oxygen meter this morning. I did calibrate it initially.
Take it for what it’s worth.
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What is it with no air added? 
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They started at 5.1 ppm before any air was started
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What air stone? Blue? Those are considered best.
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It’s a great one.
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That’s what I have used for years. I tried some bright blue ones they were good, but extremely fragile.
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