The first thing I did was try to establish some areas where the copepods could live and breed that the mandarin couldn't get to. My tank is a 20L, with 20 lbs live sand, and about 15 lbs of live rock arranged in one single reef that runs the length of the tank. In my other tank, I have a crushed coral substrate instead of sand. I noticed that a lot of copepods lived in the crushed coral. So I added a pile of crushed coral to the back of the tank where it wouldn't be visable. I also added a couple of piles of attractive shells near the base of the reef. I also rearranged the reef to include some areas where the goby could not enter. I can't prove that this helped sustain the population, but I believe it did.
Immediately after the fish was added to my tank, I began trying small amounts of other foods to try to find something else it would eat. I used a turkey baster and squirted the foods right towards the goby. For the first month or so, it wouldn't eat anything. I assume it was eating copepods during the night, but I never actually saw it eat. After roughly a month, I finally saw it eat a couple live brine shrimp after I squirted them to it. After another month, the goby was eating live brine shrimp and freeze dried krill.
About two years later, the goby is still alive in my tank. I belive that keeping this goby with other slow moving, peaceful fish has helped aid my success. The other occupants include 1 Threadfin goby, 1 Pajama Cardinal, and 2 ocellaris clowns. The mandarin still prefers live brine shrimp, but will take small freeze dried krill, and once in a while will eat a flake or two of flake food.