they arent bad for your tank untill they die. I had a flat worm problem, you need to get a product called Flat Worm Exit. You can sit there and watch them die, there are so many in your tank that you cant see its rediculous. BUT when they die they give off a toxic chemical into the water, soooooo if you have an old filter or something laying around you need to get a filter bag and fill it full of charcol (carbon) to take out the toxin, the very next day you're gonna need to do atleast a 20% water change, I did a 30 % in mine. You may have to treat your tank 2 times to get rid of all of them but thats the only way to get them, or you can buy flat worm eating nudi branchs, they die after all the flat worms are gone. most of the time they will come in on corals the best thing to do is dip them in R/O fresh water flat worms hate fresh water and they will all fall off as soon as you do that. All in All they wont harm your tank, they are just ugly to look at.