This is called the 'Escape response', and is simply a desire to swim into more favourable waters, which would be possible in the sea. The response will also occur in other unsuitable conditions like reduced salinity, excessively high temperatures, and ammonia-polluted waters. Several important points are often lost in a wealth of technical information in the more popular fishkeeping texts. As the temperature rises the oxygen content of the water decreases. Fish have varying abilities to tolerate reduced oxygen levels, larger fast moving fish are generally less adaptive than sedentary shore dwelling species.
Many invertebrates can tolerate lower oxygen levels than fish.
Oxygen enters the aquarium at the surface of the water, from the surrounding air. Aquariums inevitably become 'eutrophic' with age. The end products of the 'Nitrogen Cycle', the nutrients provide sustenance for growths of algae, and a combination of events leading to an increase in organic wastes. The 'biomass' of the aquarium increases with the length of time the aquarium has been established. Maximum levels of dissolved oxygen at different temperatures.