A large range of possibilities exist when there is the only one symptom of fast breathing. It ranges from the onset of disease, poisoning, and dwindling health to something as simple as old age.
The obvious (and somewhat useless) answer is that it is under (one or more) stress. Trying to discover the source of it/them and eliminating or minimizing it/them is your goal. If the fish was nutritionally fed well and healthy before this symptom, then the fish can live a few weeks without eating. But if this is the onset of a disease, the fish might not have that long to fight it.
A photo would be best. Look closely at the fish; all over the fish as best you can for any signs of something wrong or different.
Provide additional information if you care to:
How old is your tank? When did it originally cycle?
What kind of system do you have (tank volume, dimensions, bio-filtration method, the equipment you use, any carbon or other chemical filtration, kind of substrate, etc.).
List all specimens & sizes in the tank (fish, inverts, corals, clams, snails, crabs, shrimp, etc.).
If you have a refugium and/or sump, what is living in it?
Do you use a quarantine tank and procedure? Did the fish have any problems or disease when you first got it? Any disease since?
Foods you use and feeding schedules.
Do you use any vitamins? Fat additives? Any elemental or other additives? Please list all.
Chemistries ââ"šÂ¬Ã¢â‚¬Å“ please give actual numbers (Ammonia, Nitrite, Nitrate, Phosphate, Silica, Ca, Alk., and any others you have)
Water parameters ââ"šÂ¬Ã¢â‚¬Å“ please give actual numbers (pH and your pH range, salinity or sp. gr. & range, temperature range)
Do you see any of the following in your system: hair algae; micro algae; cyanobacteria growths; dinoflagellate (zooxanthellae) growths; brown algae; diatom growth; slimes; colored patches on rock or substrate; etc.?
Water changes (how much and how often). Kind of artificial salt youââ"šÂ¬Ã¢"žÂ¢re using. Kind of source water you are using? Have you analyzed your source water for the above chemistries? When did you do these tests last? What is the TDS of your source water?
List what you added or taken out of your aquarium system (living, decorations, and equipment) during the past 6 weeks.