it is preferable to keep a seahorse only tank however Pete Giwojna suggests the following fish as suitable tank mates:
Compatible fishes: gobies, firefish, Ocellaris clownfish, mandarinefish, scooter blennies, cardinalfish, jawfish, linned wrasse, orchid dottyback, blackcap gramma, midas blenny
Anthias (assorted Mirolabrichthys, Pseudanthias, and Anthias sp.)
Firefish Goby (Nemateleotris magnifica)
Purple Firefish Goby (Nemateleotris decora)
Gobies (assorted small species)
Neon Goby (Gobiosoma oceanops)
Assessors (Assessor spp.)
Midas Blenny (Ecsenius midas)
High Hats (Equetus acuminatus)
Marine Betta (Calloplesiops altivelis)
Banggai or Banner cardinals (Pterapogon kauderni)
Flame cardinals (Apogon pseudomaculatus)
Pajama cardinals (Apogon nematoptera)
Pipefishes (assorted small species)
Percula clownfish (Amphiprion percula)
False percula clownfish (Amphiprion ocellaris)
Royal Grammas (Gramma loreto)
Blackcap Basslets (Gramma melacara)
Green Chromis (Chromis viridis)
Longnose Hawkfish (Oxycirrhites typus)
Six Line Wrasse (Psuedocheilinus hexataenia)
Flasher Wrasse (Paracheilinus sp.)
Fairy Wrasse (Cirrhilabrus spp.)
Scooter Blennies (Synchiropus spp.)
Green Mandarin Goby or Dragonet (Pterosynchiropus splendidus)
Psychedelic Mandarin Goby or Dragonet (Pterosynchiropus picturatus)
Orchid Dottyback (Pseudochromis fridmani) - avoid other Pseudochromis species!
Compatible fishes: gobies, firefish, Ocellaris clownfish, mandarinefish, scooter blennies, cardinalfish, jawfish, linned wrasse, orchid dottyback, blackcap gramma, midas blenny
Anthias (assorted Mirolabrichthys, Pseudanthias, and Anthias sp.)
Firefish Goby (Nemateleotris magnifica)
Purple Firefish Goby (Nemateleotris decora)
Gobies (assorted small species)
Neon Goby (Gobiosoma oceanops)
Assessors (Assessor spp.)
Midas Blenny (Ecsenius midas)
High Hats (Equetus acuminatus)
Marine Betta (Calloplesiops altivelis)
Banggai or Banner cardinals (Pterapogon kauderni)
Flame cardinals (Apogon pseudomaculatus)
Pajama cardinals (Apogon nematoptera)
Pipefishes (assorted small species)
Percula clownfish (Amphiprion percula)
False percula clownfish (Amphiprion ocellaris)
Royal Grammas (Gramma loreto)
Blackcap Basslets (Gramma melacara)
Green Chromis (Chromis viridis)
Longnose Hawkfish (Oxycirrhites typus)
Six Line Wrasse (Psuedocheilinus hexataenia)
Flasher Wrasse (Paracheilinus sp.)
Fairy Wrasse (Cirrhilabrus spp.)
Scooter Blennies (Synchiropus spp.)
Green Mandarin Goby or Dragonet (Pterosynchiropus splendidus)
Psychedelic Mandarin Goby or Dragonet (Pterosynchiropus picturatus)
Orchid Dottyback (Pseudochromis fridmani) - avoid other Pseudochromis species!