I am not sure what’s going on with your new male seahorse, or if it’s anything you should be concerned about.
The first thing that came to mind when you mentioned that your seahorse is not curling its tail but instead always holds it out straight when swimming was a possible problem with tail rot. Increasing tenderness and a progressive loss of prehensility in the tail are early indications of tail rot. As you might expect, this problem is due to an infection that attacks the tails of seahorses. The tip of the tail typically turns white and, as the infection spreads, the whiteness moves further up the tail and ulcers or open sores begin to form where the skin peels away (Giwojna, Oct. 2003).
Hobbyists usually refer to this problem as Tail Rot or White Tail Disease, but the disease is already well advanced by the time whitening or tissue erosion occurs (Giwojna, Oct. 2003). Early detection makes it much easier to get these infections under control. Some of the early indicators of a tail infection to watch for are discussed below.
The disease begins with a loss of prehensility in the very tip of the tail (Giwojna, Oct. 2003). At this stage, the seahorses can grasp large objects just fine, but cannot take hold of slender objects with a small diameter (Leslie Leddo, pers. com.). Next the loss of prehensility spreads further up the tail and the seahorses begin to act as if their tails are very tender and sensitive. They will drape their tails over objects rather than grasping onto them and begin to drag their tails behind themselves, often arching the end of their tail upward in the shape of "U" (rather than the usual "J" or tight coil) as if to lift it off the ground and keep it from touching anything (Leddo, pers. com.).
This is usually when the tip of the tail becomes white and the loss of coloration starts advancing further and further up the tail (Giwojna, Oct. 2003). At this point, the discolored skin begins to flake or lift up and open wounds and ulcers develop on the most distal portions of the tail (Giwojna, Oct. 2003). The infection attacks the underlying tissues, and the tail is gradually eaten away, often all the way to the bone, exposing the vertebrae (hence the name Tail Rot). Survivors may end up missing the last few segments of their tail (Giwojna, Oct. 2003).
If you’re new male had obvious symptoms of tail rot, then he should be treated in isolation with a powerful combination of antibiotics as soon as possible, but I don’t think that’s the case. According to your post, the seahorse has no trouble perching and using his prehensile tail to wrap around hitching posts, and you did not mention any change of coloration beginning at the end of his tail. This suggests to me that if he is having some sort of a problem, it is not with tail rot, at least at this point.
It is normal for seahorses to hold their tails straight when they are swimming if they want to go up. When seahorses are swimming and want to ascend, they will lift their heads and extend their tail out straight beneath them and hold it extended in order to shift their center of gravity and make it easier for them to rise. Likewise, when a swimming seahorse wishes to descend in the water column, it will tuck its head and curl its tail beneath it in order to shift its center of balance and make it easier to swim downward. So a swimming seahorse only curls its tail beneath its body when it is trying to go down — when it wants to rise up through the water column, it keeps its tail straight, and I am wondering if perhaps that is all that is happening in your case.
It doesn’t sound like your male is having any problems with positive buoyancy (i.e., the tendency to float) since he keeps his tail extended straight while swimming and sometimes hangs upside down when he anchors to a hitching post. So it doesn’t appear that he’s having a problem with gas bubble syndrome either.
Since he is a new seahorse and not exhibiting any obvious signs of a problem at this time, I would just keep a close eye on him for now. As long as he is eating well and shows no signs of respiratory distress or any of the other symptoms we have discussed above, then he’s probably all right. But please get back to me right away if you notice any symptoms of tail rot or buoyancy problems (either a tendency to float or a tendency to sink), and keep an eye on his breathing rate compared to the female which appears to be doing very well.
Best of luck with your new seahorses!