Well even among experienced, successful keepers you will find a variety of opinions on many aspects of seahorse keeping.
For temperature though, the majority of seahorse tanks are kept at 74° or below, but, there are also some that range from 75° to 78° and I imagine even some a bit higher than that. When you get above 74° then the nasty bacteria tend to get out of hand unless you are extreme in your husbandry and water changing protocol which needs to be fairly excessive even at 74° or a bit below. Some use expensive filtration systems to aid in keeping the system healthy but still the mechanical filters should be cleaned every 5 to 10 days.
Also, there are tanks that have survived higher temps due to the fact the hobbyist isn't as informed as others but the seahorses they purchase just happen to have personal tolerance levels to pathogens higher than the average seahorse. I would estimate the numbers of such would be fairly low thought.
See Pledosophy's post #5 in the thread at
http://reefcentral.com/forums/showthread.php?t=2274878
As for salinity, seahorses are being kept in a wide range from s.g. of 1.018 to 1.026 but most fall in the 1.023-24 range that I've seen.
Some seahorses come from waters that routinely have lowered salinity due to freshwater runoff so they experience not only a sometimes lower range, they live with a changing scenario but not usually exceeding 1.026.
