If rapid breathing is it’s only symptom, I wonder if your water flow is adequate? If you look at the top of the water, from in front of the tank, can you see ripples of water motion across the top? Air exchange (co2 out and o2 in) can only happen at the surface. I think sometimes we place so much emphasis on mellow flow in seahorse tanks we don’t get adequate flow for their needs…..
Esecially when we think about their natural habitat. Their bodies seem to be well adapted to way higher flow than we give them credit for. They are like streamlined for the flow to move round them! Consider the ebb and flow of water in a sea grass bed. We were in Belize last year, where the barrier reef makes a protected seagrass bed, and the flow over my legs walking in it is still significant.
But, rapid breathing is a symptom of parasitic, fungal or bacterial infections or high NO3. or many things. Water flow is just one thing to consider?