It's mandarin dragonette, BTW. The Mandarin is not a goby.
As others have told you, a mandarin needs a well established (1+ year) tank to survive, regardless of your tank setup.
If you are determined to keep the fish, then you have a duty to learn what kind of care you must provide. The most important food source for a mandarin is a healthy, established pod population in the tank. If you are not willing to provide that, you will kill the fish unless you invest the time to train the fish to frozen and/or pellet foods.
Here's a great post by Matt Pedersen, who is actively breeding these fish, on how to train them to frozen:
Chris, I know exactly how to do this. I've written it up several times, so pardon my brevity. I'll be happy to answer any questions you have!
#1. I put any new Synchiropus into a "Breeder Net" - it's a small net pen that you hang in the tank. This allows the Synchiropus to be isolated from the rest of the community, allowing it to settle in and not have to compete for food. This is the crucial first step...skipping this makes the rest relatively pointless!
#2. Start off with feeding ENRICHED adult live brine shrimp. Enriching is crucial...I like using Selcon for a final enrichment, and I feed the adult brine shrimp with phytoplankton I've cultured here (T-Iso, Nanno, Tetraselmis etc).
#3. What happens is that the mandarins get accustomed to taking the live brine shrimp. Having the breeder net, typically what happens is that external currents will push the live brine up against one of the sides of the container...so they get pinned there. So you have a live, wriggling brine shrimp sitting there on the "substrate", and to a Synchiropus this is a pretty natural presentaiton.
#4. Once the Synchiropus is feeding well on live brine shrimp, you start mixing in frozen enriched brine shrimp. While it doesn't wriggle, it otherwise looks the same and gets pinned to the side of the container just like the live stuff does. It may happen quick, or it may take weeks, but once you see the Synchiropus take the frozen brine a couple times, I generally quickly wean the fish off live brine.
#5. Once the fish is eating enriched frozen brine consistently, introducing new foods is simply a matter of time. The next food in the progression is frozen mysis, and FWIW, a Synchiropus will live, get fat and spawn on a diet of enriched frozen brine and mysis.
#6. Once the fish is feeding well on the foods I want to be sure it eats, the fish is ready to go into the tank at large. I've found using a "Feeding Station" (look this up in relation to Seahorses) works really well with Synchiropus (also look up Malev's Mandarin Diner). You have to watch the Dragonette to make sure it's getting what it needs and that it's found the feeding station and otherwise just not getting outcompeted. FWIW, this is probably more of a problem in a LARGER aquarium vs. a smaller one (i.e. I keep 2 Synchiropus splendidus and 2 S. stellatus in a 24 gallon cube...it works and "breaks the rules" because they're all fully trained onto prepared foods and I feed pretty heavily!) If the fish goes into the main tank and starts to get thin, you'll have to repeat the process or reconsider the community it's going into (obviously if we are talking a breeding situation with dedicated broodstock tanks, outcompetition shouldn't be an issue!)
Bottom line, once they settle in, they are pretty solid fish in my book. Obviously you need to start with healthy fish, i.e. avoid the really emmaciated specimens...they are too far gone. Look for the ones that are active and picking around the tank for food.
As far as PELLETS, my only thing with those is that it takes a LONG time. Luis made the observation that he tried an old batch and didn't have any luck..when he used a newly opened container they went right to it. Mine mandarins probably took 6 months or longer to accept pellets, whereas my Red Scooter Blennies may take them in as little as 2 week's time! Formula 1 pellets (smallest size) seem to be the ideal first pellet to offer -they're soft and have a really enticing smell. After that I've been able to get mine to also eat Formula 2 pellets and Spectrum Pellets.
Bottom line, If you take your time and dedicate the effort, you'll have no problem.
FWIW,
Matt
https://forums.saltwaterfish.com/t/364424/green-man-dragon-gobi#post_3121985
Just remember that you have to keep the mandarin well fed or it will die. As you can see above, it takes a lot of effort and patience to accomplish this task.
Mandarins are like aquatic hummingbirds - always eating.