New Contributor

Koi nibbling on black moor's tail

Hi, so I'm kinda freaking out because my koi, who has never done this before, has started going after my black moor and nibbling on his tail. I had 2 black moors and the koi for about 3 months or so now, and recently I got a pleco that I guess was sick because upon introduction it brought ick to my tank. I ended up treating the whole tank for ick and sadly both the pleco and one of my black moors died. Now the tank is all cleaned up, even bought a bigger filter just in case, but for some reason, now my koi fish just won't leave my other black moor alone. It's just the two of them in there now, and the poor black moors tail is just getting messed up. It almost looks like fun rot, but I ruled that out because I do actively see the koi going after him. I guess my question is, do I have to get rid of the koi now? Was the presence of the other fish keeping him in line? Should I try feeding them more often? Not sure what I should do about it at this point.
Esteemed Contributor

Re: Koi nibbling on black moor's tail

[ Edited ]

As Koi grow they become more likely to eat smaller tank mates. They are opportunistic feeders, but are destined to be huge strong fast fish that can easily take apart a fragile fish like a fancy goldfish. Aside from the fact that both these fish enjoy cool water temperatures and are best kept in ponds, they are not compatible. I don't know the size of your tank, but to be adequate to the needs of these fish, it should be huge-- in the hundreds of gallons.  Koi eventually become more than two feet long.  A pair of them needs a tank in the neighborhood of 200-300 gallons.  You literally cannot have enough filtration for a fish of that size. Huge frequent partial water changes will be necessary.

Moor goldfish can grow to be the size of a large orange-- a massive fish that needs no less than 40-50 gallons at a minimum.  They are slow, they don't see well. and they can't keep up with a koi at mealtime. You can feed more, but the koi will always win, get larger and have a bigger and bigger appetite.

They need separate tanks.

Both Koi and Goldfish are very substrate oriented fish, which means they enjoy grubbing through the gravel searching for food. Because of this, there is absolutely no advantage to trying to keep a plecostomus alongside them.  Plecos sometimes try to feed on the slime coat of slow-moving large bodied fish.  Eventually, a Moor will become a target of this unwanted attention. Also, Plecos can get huge as well, which creates an even greater space problem. 

Dial back. Rehome the Koi. Get a second Moor if your tank is sufficiently large (at least 29 gallons) but be careful to select a very healthy fish and quarantine the new fish for a couple weeks in a separate tank to be sure not to introduce ich. Eventually a 29 gallon tank will not be large enough for two adult goldfish.

Not everyone will agree with me, but this will solve your problem.



“The oceans are the planet's last great living wilderness, man's only remaining frontier on Earth, and perhaps his last chance to prove himself a rational species.”
― John L. Culliney
New Contributor

Re: Koi nibbling on black moor's tail

Thanks Randy. I'll honestly probably do that. I got this 20 gallon tank a few months ago on Craigslist and the koi came with it. It was really bad, the tank was in horrible condition, the water was green and filled with mold and algae, and this poor 6 years old koi has been living in it. I cleaned it up and took pics of the koi to show some people at petco and they said it was so stunted that it would never grow again. He's only about 5-6 inches long. And the black moor was a Walmart buy and is about 3inches so again, I don't think he's getting much bigger. But I'll do what's best for my fish, so I'll probably just rehome the koi then Smiley Sad
Esteemed Contributor

Re: Koi nibbling on black moor's tail

Good for you. Koi are breathtaking when well kept, but a pond is almost a requirement. 

The 20 gallon tank is on the small side, but a single moor may do okay in it.

Did you know that the world's oldest verifiable goldfish lived in a twenty gallon tank for almost forty years? It's name was "Tish" and its owners won it and a second fish at a fair.  They obviously took its care very seriously.

Oldest Goldfish

Tish the goldfish was recognized by the Guinness Book of Records as the oldest captive goldfish, 43 years after he was won as a prize at a funfair. In later life, Tish had faded from bright orange to silver, but remained healthy until shortly before his death. As of 2003, “Goldie”, also 43 years old, will be in the next edition of Guiness. Goldfish have been known to live into their 30’s at least. Life expectancies from 10 to 20 years are normal for well cared for fish.

SourceSmiley Surprisedldest Goldfish Has its Chips

“The oceans are the planet's last great living wilderness, man's only remaining frontier on Earth, and perhaps his last chance to prove himself a rational species.”
― John L. Culliney
New Contributor

Re: Koi nibbling on black moor's tail

Are they different gender?  Can you tell? Might be that they are mating.

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