I may try a wrasse... I have a mandarin and it is not helping at all.. I am just a little cautious about adding a wrasse in fear of it competing with my mandarin for pods.
How about adding a second mandarin, provided that you have a fair amount of live rocks? I like to keep fish in pairs if possible.
I tried them all these biological control of the red flatworms and in my tank mandarins were winners. Sixline wrasses are definitely more efficient but they can be really mean to others.
This method works really well. You can actually help your mandarin by sucking out some flatworms with the airline tubing with a micron bag or a woman's panty hose (my favorite tool).