For brackish puffers, as valid as for many freshwater-puffers, snails are the main food to create the regular wear and tear of the dental-plates. Otherwise they will grow constantly to bulky beaks.
The owner of a brackish puffer should pay attention to an accurate feeding, because snails die fast in the tanks of over concentrated brackish water and pollute the water highly. The puffers do not care if the snails are alive or dead so the owner could even use frozen snails. Defrost them quickly under running hand warm water and feed directly.
If there are any snails left, do not refreeze them! Once defrosted, they decompose very quickly which is dangerous for puffers. Also the puffers like strong living or frozen food like water-woodlouses, river-flea shrimps, crayfish, and mussels and so on. They also like insects. Be careful with the feeding of red gnat larvaes or tubifes. They are often taken from polluted water.
A trustworthy dealer can tell you where he got the food from.
Good red gnat larvaes may help all fish which refused food before.