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Dr. Jo
Dr. Jo, Veterinarian
Category: Pet
Satisfied Customers: 2808
Experience:  DVM from Iowa State University in 1994; actively engaged in private regular and emergency practice since that time.
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My Koi are in a pond where they have been two years. There

Customer Question

My Koi are in a pond where they have been two years. There are are eight fish. They suddenly attacked one fish until they killed it. What is going on? Joanne
Submitted: 2 years ago.
Category: Pet
Expert:  Dr. Jo replied 2 years ago.
Hello,
I'm Dr. Jo and I'm here to help you with your question about your koi. I'm so sorry you're in this difficult situation, but glad you're looking for the information you need.
You may join the conversation at any time by typing in what you want to say then clicking REPLY. That way we can chat back and forth until you're satisfied with the information I've provided and are happy to give me a positive rating.
I can understand how very upsetting this must be, especially when they have done fine in this arrangement for two years. The most common reason this happens is the fish have outgrown the tank. Please keep in mind this can happen even if the fish don't necessarily appear to be getting larger. When koi are in too small an enclosure, they get stunted. This means they don't look any larger on the outside, but their internal organs still try to grow and they suffer because of it. This type of stress from overcrowding (and also usually too much waste in the water) leads to aggression, fighting, illness and death.
Unfortunately, this is so common that I'd be inclined to think this is what may be occurring in your situation until proven otherwise. I'm sorry to be the bearer of bad news, but I feel compelled to give you an honest answer.
Koi require 250 gallons PER FISH. I know examples where this is not the case are ubiquitous. Sadly, anything less leads to problems with the fish after a couple of years.
I do hope this helps to answer your question. If your pond is well over 8x250=2000 gallons, please let me know and we'll look into other potential reasons for this aggression.
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Expert:  Dr. Jo replied 2 years ago.
Hello,
I'm just checking in to see how things are going with your koi. Please let me know if there is anything else I can do to be of assistance.
Thank you.