my seahorse is breathing rapid, hanging upside down

Sorry but it's pretty hard to help when we don't know much about the situation. What kind of set-up and size, how long have you had them, how many, is the pouch inflated, how long since you noticed the problem?
Anything else you can add in the way of info can possibly help.
In generic terms, water quality is almost always the cause of most problems in seahorse tanks.
You can test everything you have test kits for but they will NOT tell you when the water quality has degraded to the point that it is going to cause bacteria lesions or internal organ damage. There just are NO KITS available to the hobbyist that will tell you this. For that reason, seahorse tanks have to be treated PRO actively, not RE actively.
Usually this occurs over a period of time as build up of detritus/uneaten food provides the nasty bacteria with food and bedding for expansion, especially if the water is above 74°F. https://fusedjaw.com/diseasehealth/temperature-and-seahorses-a-commentary-by-dan-underwood-of-seahorse-source/ also see post 5 on http://reefcentral.com/forums/showthread.php?t=2274878
Until we have sufficient information, I can only suggest that if the pouch is inflated, do a pouch evacuation. https://fusedjaw.com/diseasehealth/is-your-seahorse-floating-seahorse-pouch-evac-video/
If not, proceed to doing a fresh water dip https://fusedjaw.com/diseasehealth/how-to-give-a-seahorse-a-freshwater-dip/ followed by placement in a hospital tank with new water matching salinity and temperature of the display tank and using open ended airline for aeration and water movement.
When doing the FWD, look for thrashing around, especially at first as if parasites are a big problem they will start exploding out of and off the seahorse and you can probably see them in the container.
In the hospital tank, depending on what else you can tell us, you may need to use antibiotics like tri-sulpha and or Furan II with my first choice being Furan II but you can use tri-sulpha at the same time for better results sometimes. If the problem is internal organ damage you will possibly need to get Diamox if frequent water changes to keep the hospital tank water extremely clean doesn't work on it's own.
Now, if water quality has indeed caused a problem then you need to do a major cleaning of the system complete with a major water change followed by more frequent and larger changes in the future, coupled with a more severe husbandry protocol.

appreciate the quick response, I am so sad, he didnt make it. 
its a 65 gal, been running since december, with what i believe to be great water quality, going by the best test kit (not cheapo one) that we could get. everyone else in tank is great,(shrimp are growing, as is firefish and crabs) the other seahorse is growing, he was not, so I just dont know.
I did do a FWD and for a few hours after, he seemed to act much better. we keep it clean and add distilled water when cleaning, temp always spot on.
going to order more from same company.
wish me luck. we wanted to shoot for 3 pair as the tank is upright and has plenty of room

Sorry to hear the bad news.
You believe the water quality to be great? Based on what? There ARE NO TEST KITS available to the hobbyist to be able to check for the kind of water quality seahorses need to combat nasty bacteria that causes lesions and organ failure, just for two most prevalent things. Testing for ammonia, nitrite, and nitrates etc, will NOT tell you when the bacteria are about to take over although temperatures above 74°F can indicate the potential once quality has degraded. They can tell you when something else is or going to be a problem as in ammonia presence that will at the least burn the gills and at worst kill the seahorses.
Seahorses are NOT like other marine fish (or shrimp/snails) and are susceptible to infections that other marine fish normally have some immunity to.
Like people, seahorses have varying immunity to things and this one was obviously more immune than the other one. However, if you don't find what, and correct the problem the other one might possibly fall victim to further down the line.
Another thing to consider is that many seahorses have problems dealing with pathogens introduced to them by other tank mates, mainly if they haven't grown up with the exposure to those particular pathogens.

I've been looking for these monkeys for a long time. It really helped me a lot.