velvet nudibranch eats these things - and ONLY these things, so if you don't have them, you can't keep one. They also only live for about six months best case, so I would go this route before I put poison in my tank, but that is just me.
I have been looking for one of these for a few months- it seems you can find them mail order but not arround here in my LFS
. I have the SAME things in my nano and they are a pain in the A%^. It seems I can't ever get all of them out of there no matter how many times I siphon and pull live rock chunks out and rinse.
Also, mandrins and six line wrasse will eat them, but not ALL the time, it is a hit-or-miss thing I have been told. velvet nudibranch are the only thing out there that will ONLY eat them and leave your tank alone.