Clownfish do indeed meet many of the criteria for suitable tankmates, but should generally be regarded with caution (Giwojna, Feb. 2004). Most species, such as Tomato Clowns (Amphiprion frenatus), Maroon Clowns (Premnas biaculeatus), and Skunk Clownfish are surprisingly aggressive and territorial, and should be shunned on that basis. Others do best when keep with anemones, which are a threat to seahorses. All clownfish are prone to Brooklynella and Marine Velvet (Amyloodinium), and should be considered Ich (Cryptocaryon irritans) magnets as well (Giwojna, Feb. 2004). The only species I would recommend as companions for seahorses are Percula Clowns (Amphiprion percula) and False Percula Clownfish (A. ocellaris), and then only after a rigorous quarantine period (Giwojna, Feb. 2004). Captive-bred specimens are best.
If you’re looking for flashy, reef-six companions for your seahorses, I would suggest the Firefish Goby (Nemateleotris magnifica) or better yet the Purple Firefish Goby (Nemateleotris decora). Both are passive planktonivores with brilliant colors and fancy finnage that are not at all shy about showing themselves in the open water.
The Royal Gramma (Gramma loreto) and the closely related Blackcap Basslet (Gramma melacara) also boasts vivid coloration and a generally gentle nature, but are a bit more on the shy side. They may not show themselves as openly as clownfish or Firefish.
The gorgeous Orchid Dottyback (Pseudochromis fridmani) also does well with seahorses and is reef-safe. (Avoid other Pseudochromis species, however, which can be very pugnacious and territorial.)
Decorative shrimp such as Peppermint Shrimp (Lysmata wurdemanni), Fire Shrimp (Lysmata debelius), and/or Scarlet Cleaner Shrimp or Skunk Cleaner Shrimp (Lysmata amboinensis) are also good tankmates that will add a splash of color and activity to the seahorse tank.
Peppermint shrimp (Lysmata wurdemanni) are a favorite with seahorse keepers because they eat Aiptasia rock anemones, and both the peppermints and Scarlet cleaner shrimp (Lysmata amboinensis) will perform another useful service by grooming the seahorses and cleaning them of ectoparasites. As an added bonus, they reproduce regularly in the aquarium, producing swarms of larval nauplii that the seahorses love to eat. And the cleaner shrimp make excellent scavengers as well.
Just remember, it is important to select the largest possible cleaner shrimp for your seahorse tank(s). Seahorses will actively hunt small cleaner shrimp and they are quite capable of killing shrimp that are far too big to swallow whole, so the cleaners need to be large enough that they are not regarded as potential prey.
Another thing to keep in mind when introducing cleaner shrimp to your aquarium is that they are more sensitive to water quality and rapid changes in pH, temperature, or salinity than fishes are, meaning the shrimp need to be acclimated more carefully and gradually. Whereas drip acclimation should be avoided for seahorses that have been on the shipping bag for 24 hours or more, it is the perfect way to acclimate delicate shrimp from your LFS. They will do best it drip acclimated to the new aquarium over a period of several hours to allow them to adjust to any differences in the water parameters very gradually.
Shrimp that are introduced to a new aquarium too abruptly will not flourish and are liable to die within a day or two from the stress of acclimation, unable to adjust to any significant differences in pH or salinity, or they simply fail to thrive and expire a week or two later for no apparent reason. If the shock is too great, they will autotomize, dropping legs, claws and/or antennae immediately upon being introduced to the new aquarium conditions.
Whatever tankmates you ultimately decide upon, Nigel, remember there are two precautions that should always be observed when contemplating keeping seahorses with other fishes:
(1) All fishes that are intended as tankmates for seahorses MUST be quarantined first without exception. For the same reasons we discussed earlier with regard to wild-caught seahorses, any fish you bring home from your LFS is a potential disease vector for all manner of nasty pathogens and parasites, and you need to take every possible precaution to prevent these from being introduced to your display tank.
(2) If you are new to seahorses, you will be much better off sticking to a species tank rather than attempting to keep them in a mixed community. Beginners are well advised to keep things as simple as possible while they learn the ropes, and introducing other fishes and invertebrates tankmates complicates feeding and carries new risks that inexperienced seahorse keepers are ill-equipped to cope with. Get some firsthand experience with seahorses before you consider adding any tankmates other than a cleanup crew.
Best of luck with your seahorses and their tankmates, Nigel!