Hi,
These worms are survivors. They have chemicals in their tissues which are distasteful or poisonous to most predators, hence, most predators don't eat them. The small slug (it is not a nudibranch), Chelidonura varians, that eats them is, indeed, a good predator upon them. Unfortunately, it cannot eradicate them. Like all predators of this nature, it will attack and eat the worms until the worm population is so low that the slugs can't find enough to stay alive. The slugs then starve and die. However, there are always enough worms left to repopulate the system. The moral is, "Don't waste your time and money purchasing the slugs."
Many folks seem to have had success using the "Flatworm exit." However, if it is used and the worms return, it is quite likely that you have inadvertently selected for those worms that are largely or partially immune to its actions. Repeated use of the poison with some worm survival will result in a population that is likely largely immune to it.
Virtually no fish are likely to eat the worms consistently.
The worms have zooxanthellae in them, similar to corals, and really need very little in the way of food; in addition to their normal food - they actually eat diatoms and microalgae - they can subsist upon dissolved nutrients.
To control them, I suggest the following course of action:
1)Siphon them out, as many as you can - daily.
2)Do regular and large water changes to reduce the amount of dissolved nutrients in the system.
3)Increase filtration - also to reduce the amount of dissolved nutrients.
4)Try to get some macroalgae going in the system. These algae will suck some of the nutrients that the worms' zooxanthellae can use, and will help slow down the growth of the worms. You will need to periodically harvest and discard excess algae.
5)Reduce to the bare minimum, the light intensity and photoperiod.
These actions taken together should help control the worms. You may or many not be able to eradicate them in this manner, but they should be brought down to a more manageable level.
