Although a picture would help, the symptoms you're describing are pretty definate - your soldierfish probably has mechanically induced exophthalmia - popeye. There are two known causes for this; supersaturation of the aquarium water with gas (unlikely because if that was the case, most/all fish would be affected, or at least both eyes of the soldierfish would have the problem) - more likely, trauma to the fish's head or eye. This can most easily happen when a fish is netted and moved, but there are cases where they just hit something in the tank and it starts up. Soldierfish are very prone to this.
There is no treatment other than using a recompression tank (that some public aquariums do) or simply waiting - these problems often resolve on their own after a few weeks. There is nothing in your post that suggests to me that you should add medication. Likewise, some people advocate catching the fish and using a hypodermic syringe to remove the gas, but since you have to catch the fish to do this, and usually have to do this many times, it is a losing propisition, as each time you catch the fish,. you increase the chance for more damage.
Jay