In most cases, issues affecting marine fish—diseases, behavioral issues, compatibility concerns, etc.—should be addressed and rectified as quickly as possible, lest small problems transform into much bigger ones. But there are some circumstances in which the best course of action is to take no action at all—or at least take a wait-and-see approach. Here are a few examples:
Minor mechanical injuries
Given excellent water quality, fish can recover from minor injuries with surprising rapidity, sometimes within just a matter of days. Here I’m thinking in terms of mild wounds caused by physical trauma, such as a torn fin, bodily scrape, or single bulging eye (bilateral exophthalmia, in which both eyes bulge, is not typically caused by mechanical trauma and may require active treatment). Common causes of such injuries are netting, aggressive interaction with tankmates, and dashing into the rockwork after being startled or chased.
As long as any stressors that may have precipitated the trauma are eliminated (e.g., a bullying tankmate has been removed) and water conditions are optimal, fish with these types of injuries can usually be left in the display tank to recover under close observation.
Wedged in the rockwork
When frightened or newly introduced to an aquarium, certain fish are apt to wedge themselves into crevices in the rockwork or other tight spots in an effort to evade whatever dangers they perceive might threaten them. Triggers are especially prone to this behavior, but I’ve seen all kinds of fishes do it over the years. When this occurs, the hobbyist may fear the fish has gotten wedged so firmly that it can’t get out without intervention. Thus, the temptation may arise to pull apart the rocks and liberate the “trapped” specimen.
However, in most such cases, the fish are perfectly capable of extricating themselves and will do so after they’ve had a chance to calm down. Don’t add stress on top of stress by tearing apart the fish’s hiding place.
As Jay Hemdal notes in his book The Salt Smart Guide to Preventing, Diagnosing, and Treating Diseases of Marine Fishes, this viral disease, which causes off-white to gray, cauliflower-like nodules to form on the infected fish, is usually self-limiting and remission almost always occurs despite—not because of—various treatment efforts. Jay’s advice for most cases of lymphocystis is to let the disease run its course.
Petty territorial disputes
Even in a relatively peaceful fish community, there are bound to be at least occasional aggressive interactions among specimens. As long as these don’t progress much beyond displays of bluster or brief chasing and innocuous nipping every now and then, you don’t need to be overly concerned about them—especially if both/all specimens involved seem able to dish it out as well as they take it. However, do be prepared to intervene in the event that the hostility ramps up to become a source of chronic stress or potential source of injury to one or more individuals.
New fish fasting
It’s not at all uncommon for fish to refuse food while they’re getting acclimated to a new aquarium environment, sometimes for several days. Unless this behavior drags on beyond a few weeks, there’s usually no cause for alarm. Hobbyists in this situation sometimes try to force the issue by “flooding the zone” with food multiple times throughout the day. All this approach usually achieves is a precipitous decline in water quality. If you’re patient and stay the course by offering a variety of different foods on a reasonable schedule, a finicky fish will usually come around to feeding in its own good time.