Just so some new people don't get misled, many of us keep tropical seahorses in a range of 68° to 74°F but not because the seahorses come from waters of that temperature range, and indeed, most do come from warmer waters.
The reason for it is the propensity of seahorses to fall victim to nasty bacteria like the vibrio species. As the bacteria multiply exponentially, especially above 74°, and because our tanks are not continually flowing new water, the bacteria feed and bed in the detritus of the tanks but by maintaining temperatures that are not so conducive to their spread, we alleviate some of the risk of bacterial infections. Seahorses by virtue of their selective eating habits, and, due to their expelling particulate matter through the gills upon snicking their food, create very dirty water and extreme maintenance with larger more frequent water changes will go a long way to minimize bacterial problems but still need the lower temperatures to increase the odds in our favour.