Fish are bad at telling you something is wrong until it is quite wrong. By the time behavior changes enough to be obvious, whatever started it has usually been building for a while. Learning to read the early signs is the single most useful skill a koi owner can have.
Here is the honest framework we use.
First rule: it is almost always the water
New koi owners reach for medication. Experienced ones test the water. In the overwhelming majority of cases, sick looking fish are not fighting a pathogen, they are living in conditions that have degraded, and their immune systems have dropped as a result.
Ammonia and nitrite should be zero. Not low, zero. Oxygen drops when water is warm or when circulation fails. pH swings stress fish more than a stable imperfect pH does. Chase those numbers before you chase a disease.
Treating a water quality problem with medication makes things worse, because most medications put additional load on the fish and on the biological filtration that was already struggling.
What the common signs mean
Flashing, meaning a fish turning on its side and scraping against rock or the bottom, is an irritation signal. Parasites are the classic cause, but so is poor water chemistry.
Clamped fins, held tight to the body instead of spread, is a general stress signal. Nonspecific but meaningful, especially across multiple fish.
Hanging at the surface or gulping usually means oxygen, particularly in summer heat or after a still humid night. This one is urgent. Add aeration immediately.
Sitting on the bottom, not moving in warm water is a bad sign. In cold water it is completely normal and expected.
Visible damage, meaning ulcers, red streaking in the fins, cottony growth, or white specks like grains of salt, points at a specific problem and is worth a professional look.
The seasonal traps
Spring is when most koi losses happen, and it catches people off guard because the fish came through winter fine. Their immune systems come back online more slowly than the pathogens do, so a stretch of cold water with fish that are just starting to move is the highest risk window of the year. Overfeeding in that window is a common way to lose good fish.
Fall is the mirror image. Feeding a high protein food as the water cools leaves undigested food sitting in a fish that can no longer process it. That is exactly why feeding by water temperature rather than by calendar matters so much.
Quarantine new fish. Every time.
The most common way a healthy pond becomes a sick pond is a new fish added straight from a store or another pond. Two to four weeks in a separate quarantine setup, watched, before anything joins your population. It is inconvenient and nobody wants to do it, and it will eventually save your collection.
When to call
Multiple fish showing the same sign at once, any fish with an open sore, or a sudden death with no obvious cause. Those are worth a real look rather than a guess.
Our maintenance plans include water quality monitoring, which is the version of this problem that gets caught before it becomes a problem. More context in the complete koi care guide.