The same LFS
that sold you flatworm exit to kill bristle worms? They agreed that your flatworms are the reason for the fish deaths? If so, I would question any other advice from them.
If you have a lot of bristle worms, you are overfeeding. Scavenger populations are dependent on the available food.
I don't know what's killing the fish, but I do know it's not the worm pictured. Those are not harmful to your fish.
If it's not a parameter you have a test kit for, it's something else. Temp, oxygenation, some pollutant maybe, but you can stop killing the worms.