Rampaging fan tail

[SSGMike66]

I am new to the site today, I have had my koi pond only a few months now, I had 2 koi, one fantail goldie, 1 shibunkin 2 plain goldies, and a placastomus. they were all getting along fine and then all of the sudden the fan tail went on a rampage and killed the two Koi and the placastomus. He bit all of their fins off and took chunks out of their gils.

so I tested the water and all of the level are in the Ideal range, no ammonia very low sulfates and sulfites ph 7.5 water is crystal clear, about 80 degrees. my pond is about 400 gallons, I use no chemicals other than a very little bit of salt. I has lilly pads a big filter and a spitter constantly aerating the water.

My wife thinks the the fantail might spawning but I am not sure??? can Spawning cause them to suddenly be aggressive like that.

I put the fan tail in a fish tank in the house for now. but I don't know what to do?

any suggestions?

 

[fishin4cars]

A fantail attacked and killed a koi? fantails in most cases are the most gentle of all the fish you listed, I have hear of a very few isolated attacks of Plecos sucking the slime coat and injuring goldfish and koi, Koi can get a little aggressive at times but normally just little spats between them selves when young if any at all. I think you got something else going on, If there is a female of anything trying to spawn Mr. fantail is probably chasing behind them for a caviar snack.

 

[SSGMike66]

It was the Oddest thing I ever saw, it would just dart from fish to fish biting and bumping, the fan tail is the largest of all, over twice the size of the two Koi, when I walked up to the pond he had the placastomas by the gil. So I pulled him out and isolated him but he died about a half hour later so I pulled out the fan tail and Isolated him, So now he is alone in a 5 gallon tank but he cant stay there long because she is about 6 in long and has a very fat belly, that is why my wife thinks she may have eggs. I really have no clue. I am at a loss.

 

[fishin4cars]

5 gal tank, way to small. Are you very attached to it, If not see if your dealer will swap it out, Sounds fishy to me but I've seen weirder things when keeping fish. Heard even more but in most cases you find the story is a little different once you see it first hand. Pleco was very small? May have been weak and a 6" fantail is a scavanger, chasing others isn't uncommon, they are actually schooling fish meaning they like groups. fat belly is most 6" fantails, They appear pregnant but actually the more fancy they are the harder fantails are to breed. You know a large plastic tub with several holes drilled in it for water to pass in and out of and placed in the pond would be far better holding area than a five gallon fish tank.