Clownfish are members of the damsel family, and because of this they are actually quite territorial! They will pester a seahorse and stress the horse out, and most often outcompete the horse for food. The only clownfish that is considered somewhat safe in a seahorse-centered community aquarium is the ocelaris clown, also know as a false percula clown. This fish is more docile than the other clownfish, and a slightly slower feeder.
Tangs......well, pretty much the same story! Too much tank presence, they are fast swimmers that have a habit of darting from one end of the tank to the other, and they are agressive feeders, esp. compared to horses.