It all depends on the type/size/temperament of the fish. Mandarins are the most common type of fish kept with seahorses. Some small gobies work well because they stick to themselves. Generally any fast moving or aggressive fish is not recommended. I have a pink streaked wrasse with my ponies. I would not suggest any other wrasse. 90+% of the seahorse community would say seahorse only tank; no other fish. The reason for this is because Seahorses are super picky when they eat and they take forever to actually decide to eat something. An aggressive or fast fish would eat all the food before the ponies decided on their meal. Also, you need at least 2 horses or else they will not be happy. They are social and prefer to have a friend to hangout with. Whichever type you decide, I would not keep more than one other fish with horses in a 30g. Maybe 2 fish in a 60g. And I would only keep 2 seahorses per 30g.
