seahorses are like mandarins they enjoy eating live foods such as pods
that live in the live rock and sand. they could be eating this which is the reason they could be growing, but the pods
won't last forever if you have a smaller tank. you could also try live brine shrimp and then slowly mix in frozen as the horses get used to brine. then add some mysis shrimp, i would rather get my fish used to mysis over brine, there is more nutirents in it then in the enriched brine.
its usually key to start mixing in live brine though as brine is a saltwater shrimp and present in reef worlds
as for the garlic, most fish do respond to it, i'm not sure the exact reason why, could be the smell could be the taste but it is a good way to get fish trained on a food or to even help boost their immunities