The oxygen levels would be maintained at whatever they started at.
Step-by-step explanation:
The oxygen measurements be on either side of the apparatus if there were no fish in it and the two chambers were fully sealed off from one another will be the same as the initial concentration of the oxygen as it was started.
The oxygen content of the water body decreases based on the organisms respiring in it , if there were any plankton then there would be an increase in the oxygen content , when there is no organism then it would obviously be a constant.