Koi killed several goldfish. Why?

[DDDDD13]

I Have a koi, we bought it small but it’s currently 6 inches long. It is mainly black/orange.
We had 3 goldfish, 2 were a little bit bigger than the koi, but the koi chased them and bit their tails off, so they died. I don’t know why koi killed them, we always feed them 3 times a day
We also have a smaller goldfish, but that one is completely unharmed living happily with koi… maybe its because it has the exact same color scheme as the koi?
The dead goldfish were white and orange.
The one that’s alive is black/orange, like the koi

 

[Delapool]

That’s pretty odd! I don’t think I’ve seen that. Maybe volume - what size is tank or pond?

 

[DDDDD13]

According to my grandfather, it's like 20 gallons, but I'm not sure ... the orange goldfish was killed, the small one is the one still alive, this pic is from October 1st ?

The water is fine, it gets that color from the food apparently, we change/refill it like every 2 weeks, changed it on Friday last week

 

[Autumnsky]

Hello! Glad you came. Sorry it is because of a problem.

Check this out - It will help you get an understanding of tank keeping and specifically about the nitrification cycle. The filter pad has / or will have a colony of beneficial bacteria when handled properly.
Guide to Starting a Freshwater Aquarium - Aquarium Advice

These fish are way over the normal bioload which would be able to be handled by a tank that size. These fish should be in a 75G tank. The Plecostomus / algae eater is a huge waste producer as well as the Koi and gold fish.

Often when there are unsafe water parameters in the tank, a fish will act in an aggressive way. It is probable that is what was happening. And sometimes a fish is just mean. Since it seems to live with the algae eater, it must not be too mean.

Here is a thread which is really helpful in understanding.
http://www.aquariumadvice.com/forum...uire-big-tanks-visual-perspective-265871.html

 

[DDDDD13]

Hello! Glad you came. Sorry it is because of a problem.

Often when there are unsafe water parameters in the tank, a fish will act in an aggressive way. It is probable that is what was happening.

But...the pleco has lived there for 16 years (I'm serious), so...I guess the water is pretty safe?

And, the tank now contains only 4 fish: the pleco, koi, the small goldfish and the little pink/orange fish, is that a good amount of fish for the tank?

 

[Autumnsky]

If the Pleco is a common Pleco it should have grown to over 12 inches with a head big as a large hand. And the Koi would get to a huge size as well.

The fish can live in less than ideal circumstances, Common Plecos are especially tough, but t.hat does not make it good for them.

Did you see the pic of the fancy GF from the thread? In one year a quality fancy GF can grow to an enormous size with excellent care and huge water changes and super nutritious food.

So if you were to do a liquid water test on the tank you would likely see that it isn't in ideal parameters with waste producers like that in a 20G tank.

When the fish you had longer term got new addition fish it strained the tank and made it harder for the small space to handle the added waste, with feeding the new fish too.

Please read up on the getting started thread and learn about the nitrification cycle,

Do not throw away the filter material or wash it in tap water. The beneficial bacteria help process the waste.

Increase water changes.

The amount of fish are excessive for the tank space. This isn't an attempt to try and ruin things for you, it is a try to help you keep these fish in appropriate conditions and not have them die off.

A gold fish tank for one KOI would be more like a 90 gallon tank because Koi get enormous in the correct conditions. They like to swim, they eat frequently and have lots of waste.

The Pleco you have - if it is a common Pleco, should be more than 2x the size it is. But isn't because the small tank it is in.

One fancy Goldfish should be in a 20G minimum, but properly cared for a 40 Gallon would be more appropriate for a fish which gets more than the size of a softball and swims around and grazes all day for food.

People do not always realize what conditions would be best for the fish, it can be hard to believe and understand when, as you said, the Pleco is 16 years old and still alive.

This is not the good conditions the fish need.

 

[Delapool]

As above unfortunately - as an example - a 5 gallon tank will happily suit a single betta fighting fish and maybe a snail or so.