Sometimes you can have a tank running for awhile and not see them and if your tank parameters go out of whack they can just pop up everywhere.
I had them in a small system once and used FWE and it stressed the tank like crazy. I had some fish and coral loss. The population was extremely high and the water volume was small (about 40-50 gallons). They were everywhere!
When I started my new tank I fresh water dipped everything and they still showed up a few months later. I then slowly added fish, scooter blenny, lawnmower blenny, 6 line, and mandarin. They slowly brought the population down. I kept all tank parameters in check.
After about a year they were still around but only on 2 or 3 of my large pieces of rock. They were near the bottom of the tank and I didn't want to disturb the tank to fresh water dip the rocks. I ran FWE and was diligent about squirting around and in the rocks. This time my water volume is greater (about 140 gallons of water volume) and had a much smaller population. I had no coral loss or fish loss. Ran carbon, and did a few good sized water changes. And I now have not seen one flat worm for at least 8 months.
I will now dip everything with FWE before I add anything.
I think if you have a low enough population you should be ok to run FWE as a precaution. I personally would just wait to see if any show up. You can leave a flashlight on the glass at night and they should be attracted to the light. If you see any there in the morning then you know you still have them.
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