It sounds like your new female has developed a problem with positive buoyancy and is spending a lot of time right at the top of the aquarium because it requires too much energy for her to continually fight against the tendency to float, which she has to do in order to stay down at the bottom of the tank.
When a female seahorse is having a problem with positive buoyancy, it is almost always due to either a hyperinflated swimbladder or the result of internal gas bubble syndrome (GBS), in which gas builds up within the coelom or abdominal cavity of the seahorse.
Female seahorses are quite resistant to gas bubble syndrome (males are predisposed to problems with GBS because of their physiologically dynamic brood pouch, with its placenta-like, heavily vascularized tissue), and when internal GBS has progressed to the point that the seahorse has positive buoyancy problems, the abdominal cavity is typically quite swollen and distended, but you mentioned that your new female is not bloated at all, so I’m confident we can rule out GBS, Carol.
That makes me think that your new female is most likely experiencing buoyancy problems due to a hyperinflated gas bladder (i.e., swimbladder), which is a problem that normally corrects itself over a period of a few days. There is a special oval-shaped area on the surface of the swimbladder that is heavily vascularized and reabsorbs excess gas, but this is a relatively slow process that takes a while to accomplish.
As in many other bony fishes, the seahorse’s gas bladder functions as a swim bladder, providing the lift needed to give them neutral buoyancy. In essence, the swim bladder is a gas-filled bag used to regulate buoyancy. Because the seahorse’s armor-plated body is quite heavy, this organ is large in Hippocampus and extends well down into the body cavity along the dorsal boundary
When the swimbladder is inflated with just the right amount of gas, the buoyancy provided by this gasbag exactly cancels out the pull of gravity, and the seahorse will neither tend to to float nor tend to sink. This condition is known as neutral buoyancy, and it makes it very easy for the seahorse to swim and maneuver almost effortlessly. But when the swimbladder is over inflated with gas, the seahorse will have positive buoyancy and must exert a lot of energy when swimming in order to counteract the tendency to float. The heavily vascularized oval of the gas bladder will gradually reabsorb excess gas to restore neutral buoyancy. And if the swimbladder is underinflated, the seahorse has negative buoyancy and must swim hard in order to avoid sinking. In that case, the gas gland will secrete additional gas in order to further inflate the swimbladder and restore neutral buoyancy again. But these processes take a little time to work, and sometimes disease processes prevent the swimbladder from regulating itself properly.
If your female is still able to hunt and feed despite the problems caused by positive buoyancy, then you can afford to wait a few days to see if the problem resolves itself, Carol. If not, there are a few treatment options you can consider such as manually deflating the swimbladder, treating the seahorse with Diamox, or pressurizing the seahorse at a depth of 40 inches or more for a few days. However, acetazolamide (brand name Diamox) is a prescription medication that is often very difficult for the home hobbyist to obtain and fabricating a homemade decompression chamber in which to pressurize the seahorse can also be quite a project for the humble hobbyist, which means that partially deflating the swimbladder using a hypodermic is often the simplest approach.
Manually deflating the swimbladder is accomplished much like a needle aspiration, except the needle is inserted into the gas bladder rather than the pouch. This is how Dr. Marty Greenwell from the shed aquarium describes this procedure in the 2005 Seahorse Husbandry Manual:
"If a hyperinflated swimbladder is suspected, a bright light can be directed from behind the animal to visualize the location and borders of the distended organ. This is useful when attempting to deflate the bladder. The needle should be directed between the scute/plate margins for ease of penetration through the skin. The external area can be rinsed with sterile saline or a drop of triple antibiotic up all my appointments can be applied prior to penetration."
The seahorse’s swimbladder is a large, single-chambered sac that begins in the band of its neck and extends 1/3 of the length of its body cavity along the dorsal surface. It’s a large organ so if you can visualize it clearly using a bright light (just like candling an egg), releasing some of the gas to partially deflate the swimbladder is fairly straightforward and uncomplicated.
If the problem is a hyperinflated swimbladder, this simple procedure will provide your seahorse with immediate relief and cure the problem. But if you cannot make out the swimbladder clearly, Carol, then you will need to consider treating the seahorse with Diamox or pressurizing your new female at a depth of 40 inches or more. Please let me know if that becomes necessary, and I will provide you with complete instructions for administering the Diamox and/or for preparing a simple tall container that can serve as a homemade decompression chamber.
I am not familiar with the Reef Rider hybrids resulting from a cross between Hippocampus barbouri and H. angustus seahorses, Carol, so you might also consider contacting the breeder you obtained the seahorses from in order to see if this is a common problem with the hybrids and, if so, if the breeder has developed a simple treatment or method for correcting the problem when it occurs…
At any rate, I do not see anything amiss with your water quality parameters or the setup of your aquarium itself that could be contributing to this problem, judging from the information you have provided.
Best of luck resolving this problem and getting your new female hybrid seahorse back to normal again, Carol.