It sounds like you raised the salinity or specific gravity of your aquarium too rapidly, and that your seahorses are suffering from osmotic shock and dehydration as a result.
Remember, the salinity in a seahorse tank can be lowered relatively quickly without any problems, but raising the salinity upwards must be done very, very slowly to avoid dehydration.
I would suggest gradually reducing your salinity or specific gravity again over a period of several hours to provide your seahorses with some relief. This can help if they have not already suffered irreparable harm from the dehydration.
If your seahorses respond positively, and you feel you need to increase the specific gravity again at some point, be sure to take your time and raise the salinity very slowly and gradually. As I mentioned, fish can become dehydrated if the salinity is increased too rapidly, so be methodical and raise the salinity over a period of many days. Don’t hesitate to take a full week or two weeks to gradually return the specific gravity to normal levels again in small increments.
Seahorses can do very well at a relatively low specific gravity of 1.013 indefinitely and may experience fewer problems with protozoan parasites at such hyposalinity, so there is no need nor any rush to increase the specific gravity as long as your Hippocampus kuda were doing well at the reduced salinity prior to this incident.
Pete Giwojna, Ocean Rider Tech Support