It might not kill bb but it seems like there's an emerging understanding of osmotic stress and it's effect on fish. It's hard to quantify, but basically anything you put in the water can hurt a fish if you overdo it. Like humans ... Pure oxygen isn't a good idea but we also hit a point of having too much other stuff in our air. Dechlorinator is necessary, and it's good to get some stuff that helps slime coat into that product (in my limited experience anyway). It's also good for us to spray something on the kitchen counters to disinfect them but too much of even a natural cleaner will bother our lungs.
Aqueon uses something synthetic for slime coat. Seachem seems to have the best reputation for chemicals and I like the way they formulate and label too, but I'm also liking API stress coat (it addresses chlorine, heavy metals, and slime coat) and our LFS recommends it too.