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=2274878Until 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.