If your fish exhibit any of the following activities there may be cause for concern:
Head Hanging — (Not to be confused with head-banging, a rhythmic nodding action demonstrated by youth in the 70’s.) Head hanging is when one or more fish stay near the bottom of the pond with his head down. This is an indication of parasitic infestation.
Jumping — The apparent danger here is the possibility of your fish jumping out of the pond. Constant jumping may signal a problem with water quality, low pH, parasites or may be a female’s attempt at eluding persistent male suitors during spawning.
“Playing” In the Waterfall — What looks like a lot of fun for us may indicate koi and goldfish may not be getting enough dissolved oxygen. Deeper problems with gills such as fluke infestation, ammonia burn or gill disease may be present.
“Flashing” — No, not the type of flashing you’re probably thinking. When fish scratch themselves against the sides and bottom of the pond they “flash” their shiny scales in the sun. This most often indicates the presence of parasites.
A Loner — Koi and goldfish love company. The only time they want to be alone is when they don’t feel well.
Listing — Listing or floating at the surface of the water does not mean they are relaxing. This often will be exhibited by many or all of the fish in the pond and indicates problems with water quality.
Gulping — This is not to be confused with the gulping they all do when it’s time to eat. Gulping at the surface indicates low dissolved oxygen levels in the water or problems with the gills such as flukes or gill disease.
Peeling — Koi and goldfish do not molt or shed their skin like we do after we’ve been too long in the sun. Check the pH. Chances are the pH crashed and without immediate action they can all die.
Not Eating — If the weather is warm and they “normally” eat but suddenly stop there may be a problem with water quality if all of them stop eating. If it’s only one or two it would indicate possible health problems with only the few. The exception to this rule is after spawning they may not eat for a couple of days.
Whenever fish behave differently from normal there is always an underlying cause though it may only be a harmless natural event such as spawning. If your fish exhibit any of the following activities there’s very little cause for concern:
Ganging Up — This is when several fish are chasing and “beating up” a single fish. You’ll notice this happens most often in the morning or evening and with the goldfish more than the koi. This is one way to determine who are the boys and who are the girls. During spawning (and that’s what they’re doing) the boys chase the girls and literally beat the eggs out of her. Do be careful, however, that the girl doesn’t jump out of the pond during this flurry of activity.
Wallowing in the Plants — You may think they do this just to aggravate you. Bigger koi can literally destroy a plant in no time! Koi and goldfish prefer to spawn in the “shallows” and your plant may just be at the right height for them.
Playing Basketball — Koi especially have been known to play basketball with small rocks often re-locating them to the bottom of the pond by “eating” them then spitting them back out. This is perfectly normal behavior for a fish.
Not Eating After Spawning — If there is foam on the water and the fish have spent all morning huddled in a plant or at the shallow end of the pond and turn their noses up at the food you offer them, chances are they’ve already had their breakfast — eggs. Koi and goldfish consider their own eggs a delicacy and all take part in the smorgasbord that follows the fertilization of the eggs.