Koi fish eating alive another fish HELP!

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Tank size: 20Liter
pH: N/A
ammonia: N/A
nitrite:N/A
nitrate:N/A
kH:N/A
gH:N/A
tank temp: 25c

Fish Symptoms (include full description including lesion, color, location, fish behavior):

I have two Koi fish that my girlfriend gift me about a year ago, two days ago i notice that one of this fish had a bend spine, but there was no issue with that the fish was fine, but just a minutes ago i notice that it was swimming like crazy moving a lot, and then i realized that the other fish was biting and eating the other fish fins, i quickly remove the sick fish and put it in the water, it seems that it has blood in the fins, what should i do with the bitten fish?, when it tries to swim does like a backflip maybe it can't swim, how do i treat the sick fish?

Volume and Frequency of water changes: 19l Water change every 2 weeks

Chemical Additives or Media in your tank: Liquid bacteria nitro bacter and a small pump, 3 decor items

Tank inhabitants:
Two koi fish
Recent additions to your tank (living or decoration): NONE

Exposure to chemicals: None

Digital photo (include if possible):
The photos of the tank are from yesterday

Can you test Ph, ammonia, nitrite and nitrate we need that to help you.

@Colin_T @Deanasue @Byron

Why are all of your water parameters labeled "N/A"? Is the other fish actually eating him? Or is it just chasing him?

PheonixKingZ said:

Why are all of your water parameters labeled "N/A"? Is the other fish actually eating him? Or is it just chasing him?

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Probably because the OP does not currently have a way of testing

Sound like one fish is nipping the others fins which is different than eating. Good that you have separated them. Problem may be that the tank is to small now for Koi they grow to be large fish.

I raised 3 koi in a 55 gallon tank years ago after several years they grew large enough that I gave them to a friend with a pond so they would have a better life. From the picture it appears you fish may be starting to feel stress from crowding

JuiceBox52 said:

Probably because the OP does not currently have a way of testing

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I was asking @ChrisR77 the question, because we would never know. we can guess all night, but we won't get an actual answer.

Retired Viking said:

I raised 3 koi in a 55 gallon tank years ago after several years they grew large enough that I gave them to a friend with a pond so they would have a better life. From the picture it appears you fish may be starting to feel stress from crowding

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I would defiantly say that, the tank in the picture looks way to crowed and dirty.

How often do you do water changes and how much %. Fish in crowded dirty tanks become stress which can lead to aggressive behavior and sick/dead fish.

For fin damage keep him in very clean water. Fins usually grow back fully but you must keep him clean so he does not develop a disease or fin rot.

Sorry for all the questions but it helps us decide what needs to be done to help your fish, You did good in separating the fish. Can you run an airline from your tank to the bucket and have an air stone so he can stay in there for awhile or do you have somewhere else to keep him?

As far as the bent spine is concerned that may also be an injury suffered from these fish fighting. There are other possibilities but that is my opinion. Please do a major water change to the tank like 75%.

N/A usually means not available or not applicable. In this case I am guessing not available because the OP doesn't have test kits.

This issue is not water quality related so ammonia, nitrite, and nitrate have nothing to do with it.

It is not caused by a disease either.

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Can the fish with the bent back still move its tail?

It looks like the fish has broken its back. This is most likely caused by someone dropping something on the fish.

There are no organs in that area that would pop and cause that section of the body to bend upwards.

If the fish can't use its tail, I would euthanise it.

When you do a water change, how do you do that?

If you are catching the fish out and moving them into a container and then draining the tank and refilling it, stop doing that. Get yourself a basic model gravel cleaner like the one in the following link. Use the gravel cleaner to clean the gunk out of the gravel and remove some of the water from the tank. Then refill the tank with water that is free of chlorine/ chloramine.

If you don't have a gravel cleaner and can't get one due to the virus shutting shops, you can make one from a 1, 1.5 or 2 litre plastic drink bottle and a garden hose.

Use a pair of scissors and cut the bottom off a plastic bottle. Throw the bottom piece away.

Remove the cap and plastic ring from the top of the bottle and throw those 2 bits away.

Put a garden in the top of the bottle and put the other end of the hose out on the lawn or in a bucket. Use the bottle to gravel clean and drain the water out of the tank.

Remove about 75% of the tank water once a week and replace it with clean water that is free of chlorine/ chloramine. Leave the fish in the tank when you do this.

PheonixKingZ said:

I was asking @ChrisR77 the question, because we would never know. we can guess all night, but we won't get an actual answer.

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Yes that's the reason why and with all this coronavirus chaos it takes 2 weeks for a test kit to arrive