All the biological controls are very hit and miss. So don't get an animal unless you really want it in your tank. The slugs have a very poor shipping and survival record.
I have used the greenex treatment in the past with few problems (do a search on flatworms and greenex), but others have had major losses. There is a new product called Flatworm exit, that some have used with good results, while others have nuked their tanks.
In chemical treatements, like greenex and flatworm exit, a major problem seems to be the posionous juice contained in the flatworms themselves, as it posions the water. If you have large amounts of flatworms, and you opt to go the chemical route, make sure you have a 50% waterchange ready and a cinister filter loader with carbon ready to go - passive carbon filtering won't do the trick.
Currently I have decided not to go the chemical route because the risks are too great. Flatworms may be ugly, but they really don't pose a threat to your tank if you keep ther numbers down by siphoning them out every other week or so. Take some airline tubing and attatch a piece of ridgid tubing to it. Then suck out the flatworms. It is pretty easy and you can get rid of a lot of them in a very short time. Be ready for the tubing to clog with detritus.
Lately I have also been experimenting with killing the worms with Kalkwater. I mix some up and suck out some of the water and sediment with a turkey baster. With the circulation pumps off I squirt and let it settle over areas with flatworms. It seems to kill them without hurting anything else in the tank.
You will hear a whole lot about how to get rid of flatworms, but no one is really sure what works, or even about the lifecycle of the flatworm. Low tem, high alk, high ph, etc have all be thought to kill off the flatworms, but the evidence is all ancedotal. Along with my Kalk experiment above, I have also been dosing more Kalk lately, and it seems the population has gone down, but there is no way to tell if the two events are connected.
My best advice is to not nuke the tank - I just do think it is worth if for something that really doesn't hurt anything. Try siphoning for a while, 4 to 6 months and see if that helps, as sometimes the palgue controls itself over time. Also, if you like any of the fish preported to eat flatworms, give them a try as well.
Can you give us some info on your tank? Age, params, critters, lighting and such?
