I actually bought one of those nudibranchs once to control red flatworms (people call them planaria, but they're flatworms). It was cool watching him vacuum up the little flatworms, but in a 275 gallon tank with 9 or 10 million flatworms, there wasn't a whole lot he could do. I finally just dosed flatworm exit, and watched my tank do an impression of the sacking of Rome for an hour. I'm pretty sure the flatworm exit either killed the nudi, or it starved due to lack of food afterwards. I wouldn't be against trying it again, but the problem is supply - they're not the easiest nudibranch's to come by, and I think you'd need them in pretty large numbers to do any good. Also, I'm not sure how the food situation would work - do they even eat AEFW? If so, and you've only got a small population, it seems like the nudi might starve before he gets them all, and if you've also got a co-infestation with a preferred food source that could sustain the nudi, wouldn't it just eat those instead?
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