Most people are under the false impression that plants only add oxygen to the water. Yes they do, but not when the sun is not shining. The ‘reverse’ of photosynthesis occurs during the night and they respire, extracting oxygen and produce carbon dioxide. This is the reason why dissolved oxygen in aquatic systems are lowest just prior to sunrise.
I had been out to a job where dense growth of water lillies choked the pond and was responsible for fish deaths. To prove this, I had to get to the client’s premises just before sunrise one summer (yes this would make it 5am in the morning! That’s dedication!). Where I would normally expect a dissolved oxygen reading of ~8ppm (which is 100% saturation) the reading was 2ppm! A healthy system should have 70-100% saturation.
The web article in the link below shows it happening in a natural river system –
