Thanks for the update and the additional information about your five-gallon aquarium. Your basic setup sounds fine and the bio-wheel should do a good job of filtering the tank. A water temperature 73°F is ideal for most seahorses.
It was a very good idea to relocate your female to your pet store while your 5 gallon tank stabilizes.
Your little girl’s respiratory distress may have been due in part to the Melafix you were using to treat your blenny. I have received several anecdotal reports linking Melafix with seahorse deaths, with asphyxia being the most likely cause of death.
The active ingredient in Melafix is Maleluca tree oil, which has been known to impair the breathing of seahorses under certain circumstances for a couple of reasons. First of all, it seems to stimulate excess mucous production, and may cause the gills to be coated with a layer of slime. Secondly, it reportedly causes a drop in oxygen (O2) levels during treatment.
The drop in O2 levels is definitely a cause for concern, particularly if excess mucous production is impairing respiration by causing the gills to be coated in slime at the same time. If a thin film of this oil covers the surface of the aquarium, that could interfere with efficient gas exchange at the air/water interface, resulting in a drop in O2 levels and a build up of CO2.
Seahorses are more vulnerable to asphyxiation, and to low O2/high CO2 levels, than most fishes because of their primitive gills. Unlike most teleost (bony) fishes, which have their gills arranged in sheaves like the pages of a book, seahorses have rudimentary gill arches with small powder-puff type gill filaments. Seahorses are said to have "tufted" gills because they appear to be hemispherical clumps of tissue on stems. Their unique, lobed gill filaments (lophobranchs) are arranged in grape-like clusters and have fewer lamellae than other teleost fishes. Because of the difference in the structure and efficiency of their gills, you have to be very careful about using Melafix in a seahorse tank.
So making those water changes and getting your seahorse out of the tank that had been treating with Melafix was probably the best thing you could have done for her. Hopefully, she will recover completely now that she’s set up in a new tank at the pet store. If not, if she begins panting or huffing or showing any other signs of respiratory distress, don’t hesitate to administer a freshwater dip and a brief dip in concentrated methylene blue to assist her breathing.
Best wishes with all of your fishes, Marina!