I’m definitely not a seahorse expert, so hopefully you’ll get some more experienced responses (read: here’s a bump) but a couple things come to mind for me:
First, with the tank being plumbed into your 300, what is the water temperature? It's generally recommended to keep seahorse tanks at 70-74F*, as they are prone to bacterial issues and the cooler temp drastically reduces the reproduction of pathogenic bacteria.
Second, clownfish are generally considered to poor to downright bad tankmates for seahorses**, so - peaceful or not - it may be a good idea to remove them (and anything else that may cause the problems for the seahorses - crabs, stinging corals or nems, etc.).
Third, I know a lot of different species can be enticed to eat using live feed, so you might want to try adding some live mysis, amphipods, brine shrimp, or - if the horses are big enough - even live ghost shrimp. Of course, you would want to gut-load these before adding them to the tank (i.e. you would want to feed them a bunch of phyto before adding them).
Fourth, I'm not sure how you're spot feeding, but the method I've seen recommended uses a turkey baster to basically just squeeze the mysis out in front of the seahorses face***.