Originally Posted By: n8ly
If your oxygen decline is related to your plankton bloom, your fish have plenty of oxygen during the day because they put off oxygen, but at night that will actually cause a reverse effect and suck the oxygen out of the lake.

In this scenario, dissolved O2 is at it's lowest the first thing in the morning. If you are not at your pond then, you might not ever see the fish surface gasping. Later in the day after photo synthesis has resumed, O2 levels are higher and the fish would be (at that time) OK.

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