Sick pet fish need advice

Assalamu alaikum everyone. This might seem small but I’m worried. I’ve a fish that’s really unwell. It’s still alive but won’t last much longer. Should I put it out of its misery to stop its pain and maybe avoid spreading sickness if that’s what’s wrong? Or should I just let it pass on its own? It’s floating at the top, can’t swim, and is gasping for breath. Any guidance appreciated.