I have many similar shots. There are a couple of things you can try. One is shooting a snoot to try to light only the seahorse. Also try strobe positioning to focus light on the target and not as much on the background. I have never tried it, but I have heard of people putting a piece of plastic or such behind to get a white or black background, but a lot of times that is limited by where your subject is and you obviously want to be careful not to stress the critters out too much as it can kill them.