Gobies will eat a variety of foods. They are not specific to copepods. All gobies in my experience ate what i fed my fish, mysis, brine shimp, etc..
Mandarins and blennies eat copepods and usually devour their entire stock in months. Ive seen some live in larger tanks for about a year and a half. Some will go through their suppy in a few months. It is better to get a mandarin where the lfs
already has them eating other foods. They can be trained to eat but is very hard.
You can buy copepods and add them to your refug. With enough time they will swarm you dt.