I don't think that more co2 in the air is causing the problem I may be wrong but I disagree I think that acidifying oceans are cause by algae growth
In the last 200 years, people have removed 90% of the trees, plants and forests on the planet and they have replaced them with brick, stone, concrete and bitumen roads and buildings. In addition to this, there are lots of people using internal combustion engines (cars, trucks, motorbikes, lawn mowers, chain saws, etc) all around the world and these produce heat and CO2 as well as other toxic gasses. And there are numerous power stations producing CO2 & heat and lots of electrical appliances that produce heat.
The reduction of plants that would normally use the CO2 in the air has lead to an increased amount of CO2 and a decreased amount of O2 in the atmosphere, and this excess CO2 is going into water ways where it reduces the pH, GH & KH of the water.
The algae in the water is using as much CO2 as it can but people are producing more CO2 than the remaining terrestrial & aquatic plants can deal with. Subsequently the pH of the oceans is dropping and it will continue to drop until the CO2 levels in the water are reduced. This won't happen until the CO2 in the air is reduced.
If you measured the amount of oxygen and CO2 dissolved in a lake, how do you think the daytime levels would compare to the nighttime levels?
There are too many variables in an outdoor lake but I have tested this in a plastic storage tub that was full of algae (green water). I had a number of plastic storage containers that held between 40 & 200 litres of water. They were fertilised and developed blooms of single celled algae. Some of the containers were aerated and others weren't.
The containers all got about 16 hours of light per day and the water temperature ranged between 30 & 45C. These temperatures varied during the day and night with 45C being maximum during the day and 30C being minimum during the night. These are normal summer temperatures where I lived.
The pH of the tap water was 8.5 and there was less than 50ppm of GH and 0 KH.
I tested the containers for pH and oxygen level but not CO2 level.
The containers that were aerated had high oxygen levels day and night, the levels were at maximum for the test kit. The pH was around 8.5 and remained there day and night.
The containers that were not aerated had high (maximum for the test kit) oxygen levels during the day when there was light, but at night the oxygen levels dropped to virtually 0. The pH in these containers was on 8.5 during the day and dropped to 5.0 during the night. In the morning about an hour after sunrise, the pH started going up and by noon the pH was on 8.5 again.
I did similar tests with containers of water that did not have algae in and the aerated containers had high oxygen levels and no pH change day or night. The containers that were not aerated had low oxygen levels during the day and night and this was put down to the heat, with warm water holding less oxygen than cool water.