ovement
Stagnant water has a low level of oxygen. This is particularly right when the lower you go in the tank as there is no exchange of oxygen going on there. The water present on the surface of the tank has more oxygen but since this water is not moving, the oxygen doesn’t get to the lower part of the tank. An easy way to solve this issue is to place a good filter in the tank as it increases oxygen in the tank by making water move to the surface where the exchange of oxygen takes place. The filter also moves water from top to bottom in the tank thus results in the distribution of oxygen throughout it because water movement will increase oxygenation in the water.
Live Plants
Though it is not common, live plants can be a cause of low levels of oxygen in a fish tank. We all know that during the day plants use CO2 and give off oxygen. When the fish tank is in dark, the process is reversed and plants give out CO2 which can make harder for your fish to breathe. The clear solution to this problem is to increase lighting. You can also take out some live plants from the tank. In terms of algae, you should remove them from the water because they do the same thing as the live plants.
Chemicals
Chemicals used to treat disease or water for any reason can also have an impact on the oxygen-carrying capacity of water. Whenever you add any chemical to the water, you should read the label and ensure that it does negatively affect the levels of dissolved oxygen levels in the water.