To put it into perspective, my blenny is about 5" long but blennies are more streamlined. It adds more bio load than an invert, but less than say a 5" puffer or lionfish.
A mandarin in a 29 might not be a successful venture. How long has the tank been set up? I see you have about 40 lbs of lr
. It's recommended they have about 100 lbs of lr
and a good pod population to live off. You may find the rare specimen that eats frozen food, but that's not very common.