Perhaps the one method I've seen using bran flakes helps with extra protein. Simply mixing in fish pellets or other pet food is going to mold.
Maybe dextrose would work, I use dextrose sometimes in branch water when raising lepidoptera.
Pet foods are normally a supplement, I give my Lucanus elephus koi pellets every month when I check them, I suppose I could do it
more often though. Higher fat content is going to mold quicker.