In terms of composition, I really like #4 myself. However, I'm in the minority when I say I don't think it needs much cropping.
I asked my husband (who is a professional photographer) and here is what he had to say:
The photo has character the way it is - if you crop, you will lose the story of the tank and the space where your fishy swam through. If you want to keep this character, I suggest cropping only to fix the horizon which is off-kilter.
If you do wind up cropping, it will change the shot into something else, more like a catalogue photo. However, that's not to say that sort of photo is bad - it all depends on what you want to accomplish
Lastly, as a future composition tip, try to take pics with space in front of your fishy. It gives them breathing room. Showing where your fish is going versus where your fish has been. Without the breathing room, people subliminally feel something is wrong with the photo without knowing why. Hence why so many have suggested cropping
I hope that helps! Your fishy is beautiful and I like your photos.
Flambé (crowntail), Gleep and Bubba (Moss Ball Army)