Velvet Nudibranch is probably your best biological control mechanism, if you can screen your powerheads and overflows adequately. You definitely have a large enough population there to warrant one, I think.
I've actually just realized from looking at this picture... my tank has them as well. Ffffffffffffffffffffffffffffffffffffffff.
Not in anything like this number, nor near as big, but they ARE there.
Well. I guess I'm not selling anything out of my tank again any time soon.
&*^%!!!
However... I love nudibranches, and I already have screens I 3d-printed for my overflows, I'd just need to figure out something for my powerheads. Problem is, I don't have near enough of them to justify/sustain even a single animal...
and even then, they likely won't fully eradicate the population, but rather knock it back heavily, maybe even to the point where you don't see them at all. But, if you truly want them entirely gone, Flatworm Exit is probably your only guarantee... however I have concerns about the impact to my microfauna.
Also, keep in mind do NOT just treat the tank. Use a scraper during water change and siphon out as many as you can, several times, before using Flatworm Exit: flatworms will apparently release toxins on death, so you want the population as low as possible, and be prepared to swap out fresh carbon and do water changes.
Arrrrrrgh.