Captive bred seahorses will readily eat frozen food sources like brine shrimp, mysids etc. These are purchased from your aquarium shop in an ice cube form.
To feed out these foods you break off the required amount from the frozen block and add it to the tank (while still frozen). The block will float and as it defrosts the plankton will begin to float around in the tank (it is important to have good water flow). The seahorses will see the food moving in the water column and snick it up.
While frozen foods are adequate for nutrition it is advisable to feed your seahorses some form of live food at least weekly. Live brine shrimp can be purchased from most aquarium shops. Variety in frozen food sources also aids in nutritional levels for your seahorses.
Large seahorses will show no interest in newly hatched brine shrimp, they require the brine shrimp to be >3 mm.
A large adult seahorse can eat up to 20-30 adult brine shrimp a day. Smaller horses will of course eat less. Baby seahorses require smaller foods like newly hatched brine shrimp nauplii, copepods or rotifers.
Feeding is recommended once or twice daily. Do not overfeed as this will deteriorate your water quality. Variety of food sources is highly recommended
Remember seahorses have no teeth, they suck their food through their tubular snout, so all food must be sized accordingly. If it is too big they will not be able to eat it.
When feeding out fresh live food it is important to rinse the food in fresh sea water prior to feeding. Bacteria and other pathogens can come in with brine shrimp. By rinsing through a fine mesh net with clean water, you are minimising the chances of any bacteria being introduced to your tank.
The nutritional value of brine shrimp can deteriorate rapidly if they are not fed. Find out the day your shop gets their live food in and put your order in. Feed out as soon as possible. If you want to keep them for a few days they also must be fed.