Hi,
I have several small koi in a large tank. They have been together in the tank for almost a year now.
Recently one of the fish has taken to biting the pectoral fins of one of the others. He doesnt bother with any of the other fish, just singles out the same one & any chance he gets latches onto its fins.
The fish being picked on is in fine condition & is no smaller/weaker than any of the others.
Also, the fish doing the biting isn't particularly larger nor more dominant than any of the others.

It has been happening for about a week now & I noticed the poor fish now has damage to both pectorals..

What to do ?? Any advise appreciated.

Cheers,
Patrick.

Koi do not normally nip fins but I suggest you look into the male and female of these fish. It is more likely that one fish has signs of developing eggs and the other is harrassing it due to pheramones being given off by the pregnent one. More than likely the pregnant one is below health and may have a touch of finrot.
Easy to check
Has the koi in question got red lines in the fins? if so it has finrot, if it hasnt then there should be no problem as koi only have phalangial teeth in the back of their throats and nipping is restricted to sucking & not biting, untill you get to the back of their throats when they can take off your finger with them lol
regards

Cheers Malcolm,
These fish are only 3 - 4 inches long - can they be sexually mature at that size ?
The fish being harrassed doesn't seem to be in bad shape, no red lines & fins seem to be healing quickly but the other little bugger is still at it..

He will single out the fish & get a good mouthful & hang on for as long as he can..

Thanks again for the info,
Patrick

Maturity in koi can be deceiving.
Fish mature after 18 months to 2 1/2 years
growth by breeders 2 years 12" long
growth by stunting (inbreeding) 3"long
so you never know.
the male develops white nodules on its gill plates and looks like white spots when in season the female develops a enlarged abdomen which brings its belly & anal passage level with the pectorial fins and is almost fatter than its body.
Have a look for the signs, the desperation for a bit of the other makes us all harrassers, albeit its only spiders that eat the males afterwards.
regards

Cheers Malcolm, that might explain it - will look out for that this evening.

Will try the biting/harrassing thing on the missus too to see if it works 8)

-Patrick.

good luck
some of us have forgotten how to do that
regards