Originally Posted By: Theo Gallus
 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.


You can buy a dissolved oxygen test kit at some fish stores. you can also order them online. test the oxygen in the morning, noon and evening and you'll see a huge swing... with algae blooms even more. Algae pumps oxygen into the water and sucks up carbon dioxide during the day (it can also change the pH of your water by removing all the carbonic acid in the water, making your pH go up). At night algae reverses.. it starts using up oxygen and releasign carbon dioxide back into the water. At dawn you'll be at your lowest amount of dissolved oxygen and you pH will be it's lowest as well.