I’m very sorry to hear about the problems you’re having with your new seahorses.
The seahorse that has stopped eating and is floating at the top is suffering from some form of gas bubble syndrome (GBS), which is a serious condition that is eventually fatal if left untreated. If the seahorse is a male, then the gas has most likely built up within its pouch, which is a problem that is relatively simple and easy to resolve initially by releasing the trapped gas. There are a number of ways this can be accomplished, and I will go over them and explain how to proceed in some detail, if you can confirm that the floating seahorse is indeed a male.
If the floating seahorse is a female, then she is either suffering from an overinflated swim bladder or from gas building up within her coelom or abdominal cavity, which are more serious forms of GBS that are more difficult to relieve. These forms of GBS require entirely different methods of treatment, which I will be happy to explain if you can verify that the floating seahorse is a female and therefore has no pouch.
It will also be helpful if you could tell me more about your seahorse tank, Carianne. How big is the aquarium (length, width, and height)? How long has the aquarium been set up and cycled? Also, what are the current aquarium readings for the following parameters:
specific gravity or salinity
If you can provide me with a little more basic information about your seahorse tank and water chemistry, I can tell you if you need to adjust any of the aquarium parameters to make your seahorses more comfortable and assure their health and well being.
Please get back to me as soon as possible with the additional information about the gender of the floating seahorse and the seahorse tank itself, Carianne, and I will explain exactly how you should proceed in order to address these problems. If you don’t know your current aquarium readings for any reason, then just tell me if the effect in seahorse is a male or female and we can go from there…