It’s very difficult to determine whether the suspicious white patches you are worried about are a normal color phase the black seahorses may be exhibiting or whether they are problematic without at least seeing the picture of the ponies to go by. I can certainly understand why you would be concerned, since suspicious white patches and localized areas of depigmentation are often an early indication of incipient bacterial infections and certain parasites such as Costia or Uronema.
Black seahorses certainly can develop white saddles, blotches, and markings at times that are normal changes in coloration. For example, the two photographs below show the same seahorse, named Oreo, exhibiting normal color changes. As you can see, in the first photograph, Oreo is a black seahorse with a few white blazes and markings, and in the second photograph, taking just one day later, he has turned almost entirely white. The change in coloration is so dramatic that if you didn’t know better, you would think you were looking at two different seahorses. Oreo is perfectly healthy and normal in both the photographs, but has simply decided to changes coloration as seahorses are sometimes wont to do.
Photos by Leslie Leddo
On the other hand, here are some photographs of seahorses with suspicious white patches that are the result of diseases. In the first two pictures below we see discrete white oblong markings on the tails of seahorses which are due to bacterial lesions:
Photos by Leslie Leddo
In the following picture, the pale white patches seen on this Hippocampus kuda are the result of a parasitic skin infection caused by flagellates (Costia sp.) with secondary bacterial infection:
And in the following photograph, the white saddles and blotches on the yellow seahorse on the left are normal markings, but the white patch on the flank of the dark-colored seahorse on the right is due to marine ulcer disease (vibriosis) and you can already see some tissue erosion occurring at the site of the bacterial lesion:
Photo by Leslie Leddo
Marine ulcer disease is a very serious bacterial infection that requires aggressive treatment with powerful broad-spectrum antibiotics in isolation, but I think that is unlikely in your case because Vibrio is highly contagious and if you are having an outbreak in your seahorse tank it would be extremely unlikely for only the black seahorses to be affected and not the colorful ones.
You mentioned that one of the black seahorses has turned white from from the top of his belly all the way down his tail. The disease this change in coloration is most similar to is white tail disease (i.e., tail rot), but there is a very easy way to rule that out, Judy. Tail rot, or white tail 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).
But if your male with the white tail has not lost any of the prehensility in its tail and is continuing to grasp objects and hitch onto things normally, then you can be sure that the loss of coloration on his tail is not symptomatic of tail rot. If he is losing his ability to grasp things, then you can be equally certain that he is developing tail rot. In that case, let me know immediately and I will explain how to treat this condition.
Let us know if the suspicious white markings on your black seahorses resemble any of the photographs above, Judy, and if you could possibly obtain digital photographs of the seahorses with the white markings and send them to me off list ([email protected]), I would be happy to examine them and I should be able to provide you with much better advice after seeing the photographs.
In the meantime, keep a close eye on your black seahorses for any change in their behavior that could indicate a problem, such as a loss of appetite, rapid respirations or labored breathing, scratching or lethargic behavior and let me know immediately if you notice any such symptoms.
Best of luck with all of your seahorses, Judy.