All my seahorse tanks run 72F-74F. It's not that the seahorses don't like a temp over 74F, it's the fact pathogens/bacteria multiply much faster at warmer temperatures and can cause issues.
The argument I see over and over and over again on websites is tropical seahorses are found in waters 78F, 80F or 82F. That is true. However, our tiny little glass boxes of 20, 30, 50, 100 gallons of water are NOT the ocean. Pathogens/bacteria do not build up to plague proportions in the ocean due to dilution. In the home aquarium, at elevated temperatures, they quickly build up if one of the specimens carry's or was exposed to a pathogen at some time before arrival in your tank.
Seahorses can carry certain strains of Vibrio that are very active at high temps and multiply very quickly. Especially wild caught seahorses and pipefish. Most seahorse tanks are lower-flow, high organic load -due to the 2-3 time daily feeding of high-protein mysis shrimp. Perfect environment for bacteria: low flow/high temp.
By keeping the temperatures lower, you slow the multiplying of any pathogens....and coupled with regular water changes you are keeping any buildup in check.
Search any fish website or any of the dedicated seahorse websites for hobbyists keeping tanks at 76F, 78F and issues with tails turning white, flesh erosion, blisters, etc.
The first step in treatment is to lower the tank to 68F-70F. Why? To slow the progression of the pathogen. By keeping tanks on the cooler side, 72F or 74F max., you have the best chance of not allowing any issues to develop.
Tom