Just wanted to provide an update. The paradise fish is still behaving regularly. The redness that was near his fins and body has been reduced dramatically, only a very pale pink left.
After all of this and doing some research, I believe what may have happened is I had high nitrate levels in the tank. When the temperature fluctuated over the course of a couple of days, this caused stress on my fish which made the cardinal tetras particularly susceptible to the nitrates. Then, when I did a 50% water change and added the water conditioner after adding the water to the tank, the tap water contained too much chlorine or other heavy metals or contaminants which affected the remaining fish. The paradise fish survived, I am pretty sure, because he was able to breathe the air, although chlorine or other toxic substance in the tap water still showed physical distress signs on his body.