If your seahorses are trained to eat frozen food, mysis shrimp is the preferred type of food to feed. Brine shrimp, especially live, has very little (if any) nutritional value.
If your seahorses do not eat frozen mysis shrimp, you would need to feed live adult brine shrimp (not newly hatched) or ghost shrimp, etc and enrich them with vitamins and other enhancements.
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