What would most likely happen to fish in a pond with low levels of dissolved oxygen?

Fish will experience difficulty breathing

Explanation:

As you may know, oxygen is the principal element required to breath in plants, animals or any other organism. Cellular metabolism depends on that that keep us alive. Dissolved oxygen is found in the water and most of it comes from oxygen in the air that has liquified in the water. Some of it is a result of photosynthesis of aquatic plants If the fish does not get enough oxygen, as an alive animal, it will have difficulty to breath cause it is needed for it to get dissolved oxygen found in the water in order to survive.  

the atomic number will stay the same no matter how many electrons it has, so it doesn’t decrease nor increase. it’ll only change with a different number of protons