Can't tell a thing from the pic.
1. Is there a fork at one end that forms a Y shape to the body?
2. Does one end resemble cauliflower?
3. Is the body segmented?
4. What size are these?
5. Are any waving in your gravel like they only have the tip of one end buried?
6. Are any attached to the bodies of your fish?
7. Are any trailing from the edges of the fish's gills?
As far as these things swimming too fast for pics, net some of these things out, drop them on a dark surface, and try for some better pics.
BTW, even with the fuzzy pic, these do not appear to be planaria. Possibly a type of inoffensive nematode worm but, much more info and/or pics are needed.