As I understand, tangs are a no-no. False percs are controversial (they can eat all of the food before the seahorses and make them nervous with their fast movements) as are cleaner shrimp (they can pester the horses). Looks like the clam won't work either. Plus, the lights you would need on a tank with clams would likely make the temp too high for the seahorses.

However, the goby will be fine. :P

It sounds like you could add the clam to your existing tank and then start another tank just for seahorses. Seahorses themselves are expensive, but the setup is MUCH cheaper than a reef setup (no special lights or additives, you don't even need a skimmer---but buy a REALLY good filter and lots of nassarius vibex snails are a must. I stay away from crabs, but many people use them with success. Also, try to keep the temp under 75--can be hard to do around here. I am going to set up a couple of fans during the summer on mine. They can live fine in water up to 78, but disease outbreak occurs much more easily in warmer water, and you do not want to spend ungodly amounts of money and time nursing a sick seahorse. Sometimes they make it---sometimes they don't, and then you are out $$$ and depressed.) Read up on seahorse.org. LOTS of good info on there.