Use oxygen plants.
One of the main symptoms is the color of the water.
If the water is cloudy, it may already be an oxygenation problem.
It is the oxygen plants that allow you to have clear water : if you multiply them around your pool, you will have a greater supply of Co2, generated naturally by this vegetation.
Improve your pond design
There are also small economical methods to be able to create oxygen for your pond.
For example, you can, if you have a pond equipped with a filtration system, place the water outlet of your filter above just at the level of the water line.
This will produce an eddy, but it is the circulation and movement of the water at surface level that allows the water to absorb oxygen.
The water jet creates a continuous movement which allows the water to mix and stay in motion.
If you want a more grandiose touch, it is also possible to provide a waterfall : a waterfall also allows the oxygenation of the basin - and, we are not going to lie to each other, brings a certain panache to your water point.