Marie,
Of course. . .Always glad to elaborate!

I messed up.

I thought I had read that you fed their 'regular diet' as more brine shrimp. Sorry.
Sounds like your feedings are optimal. Keep up the good work.
This 'old' article on mandarins you might find interesting:
http://reefkeeping.com/issues/2005-02/nftt/index.htm
Mandarins are a rather unique fish. They hang around the bottom. I think it is because of this that Natural Selection has stepped in and helped them favor those fishes that are more resistant to diseases that hang around the sea bottom (e.g.,
Cryptocaryon irritans). But as for flukes, I've never heard they are resistant, nor have I ever heard they are immune to attack. So, even if they are resistant like they are for Marine Ich, they still need treatment.
I would say do nothing unless you notice something like them loosing weight. Unfortunately, mandarins can loose weight quickly in the aquarium, just from 'normal' starving from lack of food. They seem to often 'live on the edge' of survival in our aquariums. Then, they could be 'incubators' for more infecting organisms, if left untreated.
They will do okay in a QT in a Praziquantel (droncit) treatment.