Help with Seahorses

I have had three seahorses for about a year and they are all fine. I have one new seahorse that I got about a month ago. Now he's dying and I have two questions..... How do I humanely kill him? And should I treat my other seahorses as a preventative? His symptoms are lightning of his skin in patches, a pinhole sore in his side and what looks like the start of snout erosion. He has been doing fine until yesterday. I should also add that I notice a little bit of what looked like red algea on him the day I first brought him home. I got him at Kermit's and they told me he was tank raised (like my original three).

Thanks for any help.

JHill