All Dragonets graze almost constantly during the day. They need to have peace to feed. Other more active fish may distract a manderin from its ponderous grazing. It is best not to keep a manderin with fish that are territorial they require the freedom to search the whole tank for food items.
They also need lots of live rock to graze over and to feel secure, 75kg+ would be ideal.
You can help the manderin by adding live pods, helping keep the population up. Try to release the pods you add at the bottom of your tank or they may just get hammered by your other fish as they sink.