I've had a seemingly healthy angelfish get stuck to my filter intake before. My pleco had knocked the strainer off of the intake and I found my angelfish's face stuck in the intake. I pushed him off and replaced the strainer (also got rid of the pleco). He was fine. He is a huge, healthy angelfish now. Was he unhealthy when this happened? I don't think so. He was a lot smaller (dime-sized) than he is now.