Use oxygen plants.
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
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.
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.
Use a pond aerator
A pond aerator can supplement the necessary oxygen supply to the pond.
In addition, the aerator also makes it possible to create movement on the surface of the water, which also makes it possible to indirectly create oxygen and to moderate the temperature of the water.
Whether through natural means (such as oxygen plants) or through the integration of water jets or waterfalls, there are several methods to oxygenate your pond without pumps.