As a koi and gold fish owner, let me address a widely spread rumor about "gulping " air and koi and goldfish. They are known as "top breathers" . It is not a sign of poor water quaility as some indicate. They do this in ponds as well, and ponds that have very high oxyegen water content. Gold fish and Koi do this as a way of getting more oxyegen, but also do it while top feeding. You will see this occur more so if you have duck weeds, or fairymoss on top of the water. They feed on it! It has nothing to do with poor water quality!
I have 5 ponds all over 25,000 gallons each with an additional pond of 250,000 gallons. I have koi and goldfish and catfish and some trout all mixed in the same pond. The only fish that top breathes is the kio and the goldfish. Do not assume because they do, that that you have water quaility issues and start buying pumps and water areation devices. That is exactly what sellers of such products want you to do. They want you to buy and spend your dollar. My ponds are 20 years old. In 20 years I have lost fish to birds, to coons, and to cats. I have never found a floating fish that I could not attribute to a previuos attack by a bird or other animal.
Check all information carefully when taking advise, and I am sure there will be those that will disagree with mine.
The difference is, mine is backed by 20 years of facts , not fiction.