Koi get along well with other species of fish, as long as the other fish are not aggressive.
In a pond setting, koi will breed as a flock, or group. If sexually mature males and females are present in the flock, and if environmental conditions are met, spawning will happen spontaneously. During spawning the male koi will become extremely aggressive. Male fish will pursue the female koi around the pond, smashing into them repeatedly. This battering behavior is designed to force the eggs from the female's body. Once the female has dropped the eggs, the male will spray them with his sperm.
Goldfish and koi are both friendly fish and are not usually aggressive, although both will readily devour anything they are able to fit into their mouths. Larger koi, for example, have been known to eat small water-dwelling animals such as frogs. Goldfish easily accept newcomers into their environment, so transplanting young koi fry into a goldfish pond is fine, once the koi fry have grown to be around an inch long. Before that is too soon.
Males may chase, bully, bump and pester the females
Smaller fish kept in the pond may become food for the larger fish
I have two small longfins, 4 to 5 inches, that I recently introduced into the pond with my other, larger fish, 10 to 14 inches, a mixture of standard and longfins. The little ones tuck themselves into some "caves" that I built for them to hide in, and I will sometimes see the large koi grouped around these caves, sometimes with their noses in the nooks and crannies. I've been concerned that they are being aggressive toward the little ones, but then when I toss food in, I'll see one of the babies right alongside gobbling up the food. The other one is more timid and won't join them.We did not dissect koi specimens to find remains of the removed eyes in the gut, but it seemed as if the removed eyes were consumed by the attacker(s) since no remains were found in the tank. Jha et al. () showed that presence of food can motivate aggression in koi carp but we did not observe any cases of eye-attacking behaviour at feeding times. Although the exact stimuli for the incidence of eye-attacking behaviour was not investigated in our study, we suspect that the same feeding strategy of these species to be the one of the foremost reasons of these attacks. Both common carp and goldfish are omnivorous bottom-feeders and may compete for food. Reducing stocking density of koi or sorting job may moderate occurrence of aggressive behaviours. For future research, the effects of different stocking ratios on aggressive behaviour of koi should also be examinedJust like any other pet, Koi provide pond guys and gals everywhere with entertainment and companionship. So now your new found finned friends are chasing each other around and being rather aggressive towards each other. This violent activity may seem disturbing to us but for your Koi it is actually a natural process. No your fish have not transformed your water garden in their very own fight club; this is their way of courting each other.Sometimes female koi can fail to release their eggs. The eggs can reabsorb over time, but this is a slow process. Unreleased eggs can cause infection in the fish such as septicemia, and the results can be life-threatening. Many owners will have a professional manually remove the eggs before breeding as a safe breeding alternative because of the breeding process, which can pose harm to the fish. The breeding process can be pretty rough on the female koi; males may injure the female as several of them may go after the same female. Injuries can occur while the fish are working at expelling the eggs or during the general spawning period. Wounds and missing scales are common after the breeding process. It is important to keep a watchful eye during the spawning period because males can also become overaggressive, and fish may need to be separated.