This is just from my own humble experience - 
I had flatworms a year or so back & had never seen them before so I never knew... I tried Flatworm Exit not once, but twice. Each time it seemed like they all died, only to come back with more determination than before... being that the treatment itself can be a stressful event, (siphoning rock before & after treatment, and having 50g+ water on the ready to do a 25% water change, I decided to let nature run it's course... I got a 6 Line Wrasse and a Copperband Butterfly and within a couple of months have never seen one again. (I never actually saw either one of these fish eat a single flatworm BTW.)
Interestingly, as I have 2 tanks plumbed to a common sump, I never saw ONE in my softy tank, and never observed any of my corals being consumed.
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135G Mixed Reef. Bullet 2, 25 gal refugium, 2 X250W MH + 4X 96W PC\'s, DIY Calcium Reactor, Coralife 1/6 HP Chiller, Phosban, Tunze, 2 closed loops & SQWD\'s, Seios, Coralife 4 stage RO/DI & a bunch of other expensive gadgets... I may never retire, but I'm gonnahavahelluvanaquarium!
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