I dont know a whole lot of worms, but I've dealt with a few..
Do any of your fish appear bloated??
Have you introduced any new plants recently or anything that could have come from a pond??
Do you gravel vac??
I keep a 'pond tank' where everything in it came from a friends pond. As you can imagine, MANY MANY pest snails. But also, many many worms. And they look similar to this..
Would it be possible for them to be red worms??
Do they look segmented? (Hard to tell from the pic)
The worms in my pond tank look very similar, reddish brownish- blackish.. lol
I hope, for your fish's sake they are just red worms and not camallanus..
Best of luck!!!