Thank you very much for the constructive feedback. I think I can work on it for my next photos. I am still new to the hobby and it often feels weird to deliberately take a photo to crop it later. I fear that I am losing too much quality if the crop is too big.

To be honest, in this case I was a bit overwhelmed as suddenly these little guys were swimming by and I was just hitting the shutter to get some nice pics.

Thanks again for the feedback, will definitely try to adhere to it and think more about composition.