You can also purchase live copepods and feed these to the fish a few times a week. However copepods aren't cheap, with prices starting at about £10 for 100ml which will contain about 20 pods.
You could culture them yourself in a small seperate tank. Another option is to construct a rock pile or two in a fashion that makes it impossible for the mandarin to get into. This will create a safe breeding ground for the copepods and hopefully result in the birth of more than the dragonet can eat.
Eitherway, the copepods would ideally need to be fully established and multiplying well before a dragonet was introduced, otherwise you are pretty much endangering the fish, inviting a headache, or going to be spending £20-£40 a month on pods lol.
I'd love a dragonet. I've got just under 30kg of LR in a 180L tank. I want to get another 5kg and introduce a bottle of pods to the tank and let them establish themselves well before purchasing one.