Health Regression

[Olycius]

Hello, I currently have a 125G tank with community fish in it. Originally I had 2 Golden Killifish in this. When I purchased them, one of them had an odd shaped head sort of indented. The store I bought them kept asking if I was sure I wanted him because it did look odd when you looked at how the others looked in comparison. I noticed he was smaller than the rest in the group they had and some were picking on him. So I decided to keep him.

Fast forward 2 years and he grew full size and was the same size as the normal one I had purchased the same day I got him. But the last few months or so it changed. He began to get smaller, a lot smaller. I made sure he was eating and the water had the correct conditions. He would eat normally and did not show any signs of infections. I noticed that he started parting from the other Killi as before they would always be side by side. I went and picked up 4 more Killis during this time to see if maybe having more would help him as they were small. His coloring started to fade and he continued to get smaller until I noticed he was becoming weak. I moved him to my sick tank and he would still eat, but was about half the size he was a few months prior. I tried treating him for infections just in case I might have missed something. Sadly, he passed away a few days later. The other Killifish are doing well, and the normal one I bought at the same time is still healthy.

My question is, do you know what this could have been? He did not look like a normal killifish because of his head being indented, so I wonder if maybe that had something to do with it.

 

[Colin_T]

When you say shrinking, did the fish actually get smaller in length and width or did the others grow and that one stayed the same size?
Because fish and animals don't normally shrink as they get older. Humans appear to and its usually caused by collapsing vertebrate and a hunched posture, but fish don't have that problem because their weight is supported by the water.

If the fish actually did get smaller in length it would be very unusual and the only thing I could think of would be the part of the brain that causes growth (hypothalamus) was damaged and for whatever reason the fish started to get smaller. However, I have never heard of this happening in any animal or fish. That doesn't mean it never happens, just there have been no reports of it that I could find.

There is a link for National Geographic about fish shrinking due to warmer weather but it's completely different to this case. And there are a few other people who have asked similar questions on different forums but the fish could not be confirmed as shrinking vs the other fish had grown while one didn't.
https://blog.nationalgeographic.org/2012/11/20/warming-up-to-shrinking-fish/

Unless you have a photo of the fish when they were little and now, and there is a ruler next to them, I would have to think the one fish stopped growing and the others continued to grow. Now if you had that sort of pictures/ evidence, then this could be something really new to science and might have major ramifications for fish all over the world. I'm hoping for the latter

 

[Olycius]

When you say shrinking, did the fish actually get smaller in length and width or did the others grow and that one stayed the same size?
Because fish and animals don't normally shrink as they get older. Humans appear to and its usually caused by collapsing vertebrate and a hunched posture, but fish don't have that problem because their weight is supported by the water.

If the fish actually did get smaller in length it would be very unusual and the only thing I could think of would be the part of the brain that causes growth (hypothalamus) was damaged and for whatever reason the fish started to get smaller. However, I have never heard of this happening in any animal or fish. That doesn't mean it never happens, just there have been no reports of it that I could find.

There is a link for National Geographic about fish shrinking due to warmer weather but it's completely different to this case. And there are a few other people who have asked similar questions on different forums but the fish could not be confirmed as shrinking vs the other fish had grown while one didn't.
https://blog.nationalgeographic.org/2012/11/20/warming-up-to-shrinking-fish/

Unless you have a photo of the fish when they were little and now, and there is a ruler next to them, I would have to think the one fish stopped growing and the others continued to grow. Now if you had that sort of pictures/ evidence, then this could be something really new to science and might have major ramifications for fish all over the world. I'm hoping for the latter

He did get smaller. The normal one purchased around the same time stopped growing after about 1-1.5 years or so. They were at point the about same size. I do have videos of him throughout the two years, however with no ruler. I will link to a video I have of him. He was already beginning to get smaller in this video as he is the one at the top and you can see the normal one swimming below. He was just beginning to fade as well. The interesting thing is he was almost as wide as the bigger one at one point, and towards the end(first picture above), you can see he looked skinnier. You can see the size difference in some of the videos though(going through all of them). I did freeze him though so I can measure him now, and the videos I have of him he is next to other fish that are still alive that I can measure for reference. I will see what I can collect and reply with the results.

https://drive.google.com/file/d/1TIWuAk_Tiv5YDJb18JOghyGLHCfBKCAI/view

 

[Olycius]

He did get smaller. The normal one purchased around the same time stopped growing after about 1-1.5 years or so. They were at point the about same size. I do have videos of him throughout the two years, however with no ruler. I will link to a video I have of him. He was already beginning to get smaller in this video as he is the one at the top and you can see the normal one swimming below. He was just beginning to fade as well. The interesting thing is he was almost as wide as the bigger one at one point, and towards the end(first picture above), you can see he looked skinnier. You can see the size difference in some of the videos though(going through all of them). I did freeze him though so I can measure him now, and the videos I have of him he is next to other fish that are still alive that I can measure for reference. I will see what I can collect and reply with the results.

https://drive.google.com/file/d/1TIWuAk_Tiv5YDJb18JOghyGLHCfBKCAI/view

I found a picture when he was a lot healthier. He is the one on the right, and the normal is on the left. The normal one is the same size in this picture as in the video as well.

https://drive.google.com/file/d/1v8Pr4RyoR_73tK-X7CTF-dndSApzFu2-/view?usp=drivesdk

 

[Colin_T]

He has a definite bend in the spine in the picture. My guess is it was a tumor or genetic defect and as the spine bent he appeared to get short. I don't advise doing this but if you straightened the fish's spine, it would probably be the same size as the other male.