If you are able to adjust the settings on your phone - I can with my S21, the biggest things to help out with taking pictures of fish are going to be adjusting your shutter speed. It is written in fractions, I think default is usually like 1/50 or 1/75. If you bump that up to around 1/125 to 1/250 or so you should get some photos in a lot better focus, you can try going a little higher as well but it shouldn't make too much of a difference.
When you increase your shutter speed, you get less light into the lens due to the shorter period that it is open. You will also need to bump up your ISO a bit which will increase the light back up. You can also work on adding additional light source to where you are taking your photo - an extra light on the tank.
These are the two biggest things to take clear photos of moving objects.
Below is a picture of a few of my
guppies. You can see its a bit darker then normal. This is without any additional light to the tank other than normal, if I were to really get into taking better photos I would add an additional light source to when I take the photos.
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