So when I first got my python water changer I used the gravel vac at the end but then it fell off one day and I never got to putting it back on and was using the python without the gravel vac at the end. Well one day while changing my water my 4" SB senegaus decides he wants to check it out. He try to swim very close to the tube to investigate but instead gets stuck in the tube. I was lucky that he was just big enough to not get sucked all the way to the sink that I was using. So I ended up turning the water off and cut about a 5" piece off my python that he's stuck in. I thought that once I did that he'd be able to swim out in his own but he was really stuck. So I ended up getting a rag and pulling him out. His head was a little read and he was obviously traumatized but he eventually healed and got over it because he almost tried it again a couple of days ago.
I did that once, I had a small clown loach get sucked up at some point, I did a head count and was missing one. A few days a latter I noticed a smell in the bathroom but couldnt locate it because it was very faint. Well I had to do another water change and noticed the water was not coming out as fast as normal so I took it apart and there was the missing loach.