The video below shows what respiratory distress looks like: the fish are breathing heavily through their mouths (versus their gills) and have a tendency to hover close to the surface where oxygen levels are higher.
I sprang into action. As you can imagine, any delay could only result in casualties.
First, I checked the temperature of the water. It's possible a heater can malfunction and I wanted to ensure my fish were not cooking from the inside out. The temperature, however, hovered at 80 degrees, which is well within the right limits for angelfish.
Second, I changed 33% of the water. With my hose permanently connected to a nearby sink, it took just minutes to syphon out a third of the water and replace it with clean, fresh water. I had performed a water change just yesterday but today's water change wouldn't hurt.
Third, I added Immune Plus, which I always keep on hand. Of course I discovered this problem on a Sunday evening when all pet shops were closed so it always pays to have an arsenal of medicines on hand.
Fourth, I added amazon extract.
The Immune Plus and amazon extract are not medicinal in that they treat infections but rather they provide vitamins and the right water conditions for the fish's own immune systems to keep them healthy.
Within minutes, the fish no longer hovered near the surface. They continued breathing through their mouths for about thirty minutes before they began to breathe normally again.
What caused the problem? It's only a guess. I could have checked the nitrates, nitrites and ammonia levels but I didn't waste time - I knew no matter what the answer was, I'd still do the same steps I outlined above. Once the water settles and is circulated for an hour or so, I did water tests and all came back at zero.