Will koi and goldfish fight?

[mostlycichlids]

IMO the person should have never been told the koi could be kept in any aquarium. Koi and goldfish get along but they need space just like any other fish. If the guy said he had other goldfish I would of asked how many, and what size tank. If he replies anything less than 150g I would not of sold him anything.

To get back on the topic, I would agree that it was likley water parameters that killed the fish along with stress of adding the KOI. The Koi could very well of stressed the goldfish as they are faster swimmers and larger stronger fish.

 

[GoldLenny]

is it not even okay to keep a koi in a tank if it is still little? how fast do they grow, lps said an inch a year maybe...but even my goldies grow faster than that, i've had them for 3 months and they've gone from 1" and 1 1/2" to 3 and 4".... i was going to put a koi in my goldie tank....

Given proper space (nice big pond) and diet, Koi should grow to 1/2 of their expected adult size in 2-3 years and reach 90% of their full size in 10 years. This means they'll be 15" after 2-3 years and around 30" after 10 years. Obviously, anything less than a HUGE (multi-hundred gallons) tank would not work for a Koi for anything more than the quarantine period.

Goldfish should grow at even faster rates... 1/2 of their expected adult size in 1-2 years and full size in 4-5 years with some people getting even faster growth... probably based on genetics due to the excessive inbreeding/cross-breeding of fancy goldfish.

 

[Corvina]

koi wont kill goldfish. it was most likley the spike in ammoina that killed the goldfish. although, fish do become much more aggressive when they are in cramped conditions, but koi and goldfish like the company of one another.

Agree with jm1212.

 

[spirals]

"Customer lied" Yeah, that happens to me all the time. Once had a customer come in who wanted to buy a koi for her 10' by 10' pond (what she told me). So I scooped up the koi and sent it to its new home feeling mighty happy. Apparently she returned it the next day after it died when she put it in her 4G bio-orb ( we recognized the koi believe it or not). I was so angry...
Koi seem pretty aggressive to me, it wouldn't surprise me if one did pester a goldfish to death or out compete it for food. Koi also get so much bigger that their mere presence may harass the goldfish.