Do you have a fish that is behaving differently or looks different than it did the day before? Maybe its color is different. Maybe it’s hanging out in a place you’ve never seen it hang out before. Maybe it’s something, maybe it’s nothing. How do you determine if there is a problem or not?
That’s a tough question to answer, but the place to start is with elimination. You’ve heard “the process of elimination” before? Yep, start with trying to determine what the problem is NOT. As you eliminate possibilities, you’ll get closer to determining what, if anything, the problem might be.
Have you done anything to the tank, the water, etc?
If not, start with the water and see if something has changed that you can’t see. Is the temperature where it should be? Are the other water parameters in line (e.g., ammonia, nitrite, nitrate, pH)? If there is something out of line with your water, don’t automatically assume it will affect all tank occupants the same way and at the same time.
If all of your water parameters check out, look at the tankmates. Have you introduced any new fish into the tank? Have you removed any fish from the tank? Is the fish in question a juvenile, a sub-adult, or an adult?
Are there conspecific tankmates (i.e, tankmates of the same species)? If so, what is the gender of the fish in question and the gender of conspecifics? Is it possible the fish in question is a female and may be guarding eggs or fry? Are any tankmates spawning? If so, where is the spawning site in relation to where the fish in question tends to hang out?
Have you recently introduced something else into the tank (e.g., a decoration, a live plant)? Did you rearrange the existing tank decor or plants? Did you change the substrate?
Does the fish in question show any signs that it has been attacked by a tankmate? Other than the fish’s color, any other changes in its physical appearance that you can identify (e.g., spots, bulging eyes, growths on the body)?
These are some questions you should ask to help pinpoint what might be wrong. Then again, there may not be anything wrong. However, a sustained pattern of new behavior or physical changes other than color are typically indicative of some problem. As your fish keeping experience grows, so will your ability to not only notice a problem with your fish but also what the problem is.