You have to keep this tank sealed or the water gets slightly oxygenated by contact with air, you have to keep it in the dark and at stable temperature or other

You have to keep this tank sealed or the water gets slightly oxygenated by contact with air, you have to keep it in the dark and at stable temperature or other reactions could disrupt your experiment. How do you measure the oxygenation without introducing potential error in this context ? You can't just make an instrument appear in the middle of the tank. https://news.ycombinator.com/item?id=29429385 https://news.ycombinator.com/item?id=29429385