PLEASE thouroughly research seahorses before you decide to get one. Go to Seahorse.org for the best info.
Generally, seahorses should be kept in a species-only tank because they are very slow eaters & require a specialized diet. They will usually starve to death if kept with ANY aggressive eaters. SOME inverts, gobies & mandarins are OKAY WITH CAUTION.
Again, make sure you do extensive research & take advice only from reliable sources who are knowledgeable of SH!