Hello; 85 degrees seems high. Do your fish need temps that high?
Generally the higher the water temp, the less dissolved oxygen it can hold. Perhaps you can lower the temp. During hot weather I drop trays of ice cubes.
Check your heater. I usually have to adjust the settings down in the spring and up again in the fall. I am not familiar with the filter you named.
Do you have air bubbles? Some air bubbles should add oxygen to the water.
Sometimes when hooking up a new filter, I have heard of a bacterial bloom or mini-cycle starting. Not sure why. However, if this happened, it would cause the increase in ammonia as suggested. Definitely test and do a large water change!
This happenes to me once in a while. There are many possible reasons for this.
Not enough movement in the water - Is the top of the water still or moving?
Not enough air in the water - Turn up the air pump or add another one
Temperature could be too high
There could also be too much nitrate in the water
I find a simple water change tends to solve the problem.
get an air pump and an air stone. Or a powerhead that has an air tube on it. I agree that the high temperature will reduce the amount of dissolved O2, so it won't hurt to kick it up a notch.