I recently set up a 150 gallon freshwater aquarium. The first fish I got were 3 Bala Sharks each about 3 to 4 inches long. This was about 4 weeks ago. They did great but when they fed they would go up to the surface of the water and suck in the food, along with quite a bit of air. You could see the air bubbles go out their gills. I have had Balas before and don't remember them doing this.

About two weeks ago I added about 25 Tetras and everyone is getting along very well. The water quality is stable, no signs of disease or injuries, everything is great. Except...

Three nights ago, right after I fed them, I saw one of the Balas upside down on the bottom of the tank. His respiration seemed labored and I was ready to pronounce him a goner.

I got up the next morning expecting the worse and was very surprised to find all the Balas in apparent good health. Three days later and everyone is alive and kicking.

As the fish was in obvious distress I would like to take steps to avoid the situation in the future, but first I have to figure out what caused it.

Could the sucking in of air be in some way related?

I appreciate any thoughts you might want to throw my way!