Could always try and iodine dip. Just tank a small cup/container of tank water and put some iodine in there, not tons but not to little either. Not sure if there is a certain ratio, i usually eye it....probably not the best to do but it hasn't failed me yet. Let the zoas soak for a while. You can usually see the nasties and stuff falling off of them over time. Then just plop them back in the tank.
Its what i do anyway...