I've been keeping 2 young koi in an aquarium for a year now. They've grown bigger and accustom to me. Their tank is about 55-60 gallons, and I live in the northeast. They were from the same shop and same breed, and they also look similar so they're most likely siblings. But one has grown bigger than the other and eats more aggressively. The smaller one has recently been acting more skittish, and panics a lot.

In a result I think it may have injured its scales from the decor. I heard scales heal pretty quickly so I want to make sure I might be overreacting. But if it's not the case, I set up a quarantine tank next to the main just incase. The wounds however, have made the lager koi chase the smaller one more than usual-even biting the other's fins and the area of the wounds. They are probably too domesticated to shift to a pond, but I've also been planning to build one next year.

So are these injuries serious (images)? And can an aquarium Koi shift to a pond koi?

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2 Answers 2


I have had many Koi over the years and they are very non-combative. They won't even eat bit-size fish in the pond with them . Are there any other fish in the tank? Normally , if fish fight , the first damage is to fins. As stated , looks like disease.

  • No, there are only 2 Koi in the tank. The bigger koi has been getting more aggressive in eating since the summer started, but he's always chased the smaller one but never hurt it. Just recently I caught him trying to bite the fins entirely but lucky the smaller one was able to get away a lot. The bitten fin is still intact and okay, but it looks like it has some 'marks', meaning the pattern of the fins don't look as nice as they used to, no bleeding though. Jul 3, 2017 at 22:44
  • when one puts a new koi into the pond all of the koi forms a line and slowly swim after the new one,no stress and no fighting at all just curiosity,when koi play(this is pond speak for mating)there is a lot of splashing and showing off but still no figting or hurting eachother,if a koi gets hurt during this play it is from objects in the pond and not from the other fish. Jul 4, 2017 at 6:35

It looks to me like your koi is having an ulcer or starting to develop one, this is not unusual in koi sadly there is no really effective cure for this.

The picture showing the left side of the fish is showing many lost scales and there is an area that looks like it has sunken in a little, this why I think it might be an ulcer there but I am not sure about it. On the right side there is a wound and this might be infected quite easily.

Having koi in a indoor tank is not a good idea as a single koi needs a tank of at least 500 liters and a tank of 1000 liters is recommended. Remember a koi can easily reach 40-60 cm in four years.

Koi owners where I live do sometimes move their koi to indoor tanks in the winter if their pond is shallow and less then a meter deep.

There is normally not a problem to move koi to an outdoor pond if you let it get used to the water slowly to avoid shock.

It is best to move the koi in early summer when the temperature in the tank and in the pond is fairly similar.

I suggest you buy a book about koi like An Interpet Guide to Koi by Barry James, or a similar book.

Koi usually get very tame in a pond and they usually comes when one approaches it,and they can eat food directly from your hand.

I am sorry that I can't help you more with your fishes illness but diagnosing fish over the internet is not a very easy thing to do.

If/when you make a garden pond be sure to follow your local rules and regulations, where I live one needs to protect the pond or swimming pool so children can't drown in it, so you might need a fence around it. (The rules here is to build a fence but it doesn't say how close or far away this fence have to be so fencing can be done around the property or around the pond).

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