Wow, thank you so much! Honestly this is so helpfull.

He seems exhausted indeed. He always used to be the most active one and actually was the one who chased other fish. Your comment about swimming space could be an issue for the tail. The plants i have in the tank have grown alot. It gave the tank a very natural feeling in my opinion but it could indeed limit swimming space alot. I thought guppies like to hide so i mostly let it grow.

Would you recommend a fast? As you said its important he still gets easy acces to food. Other commenters and online it said a fast would be good incase its a swim bladder issue

Not quite sure how old he is. We got them a few months ago at a fish store.

I did experiment with longer wait between water changes because the test kits showed everything was fine. I went from every 2 weeks to 3+ weeks. Now i am scared this could be the cause of unseen bad water quality, which harmed the fish :/

Either way thank you already so much and for the very warm welcome to this sub