Koi fish eating alive another fish HELP!

[ChrisR77]

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.

I used to change 10% every week but the surfaces inside the tank started to grow a brown thing and i just replace the whole tank every 2 or 3 weeks

 

[ChrisR77]

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?

My mom had another 20L tank laying around so i put the injured fish there with this kind of filter

 

[ChrisR77]

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 there are other people in the house, see if anyone has been mucking around in the tank.

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

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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.

Aquarium Cleaning Made Easy

Aquarium cleaning should take 30 minutes every two weeks. Learn how the professionals do it in 18 easy steps.

www.about-goldfish.com

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.

Yesterday he was moving but every time he tried to he just did like a backflip, today he's been sitting at the bottom of the tank not doing anything also he didn't eat, also i add a few drops of methylene blue.
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Yes i replace the whole tank every two weeks, because i used to change 10% every week but the gravel cleaner suck almost all of the water before i could suck all the gunk

 

[Colin_T]

I would euthanise the injured fish.

 

[Barry Tetra]

I used to change 10% every week but the surfaces inside the tank started to grow a brown thing and i just replace the whole tank every 2 or 3 weeks

Dont replace the whole tank, just 75% water changes, substrate clean using gravel vacuum every week will do. About the fish, you could try add Formalin (1 drop per 10 gal please dont overdose, it can kill him) and do daily 75% water change.

 

[Colin_T]

The koi does not have a disease and formalin will not help it.