Questions about popeye and treatment?

Last night, I looked into my 20 gallon tank wand was alarmed to see that my rainbow darter had popeye on both eyes. One is significant'y worse than the other, but they're both bulging. The bad eye had a small reddish spot on the bottom of it, and there was a dark mass on top. The other eye was just bulging a bit. He has no other symptoms, and is still just as active and colorful as ever. He's also eating fine. None of the other fish have any symptoms of illness.

I immediately did a 50% water change and added two tablespoons of aquarium salt to the tank to try and draw out some of the fluid. Today, the less severe eye looks a little better, although it's still not normal. The reddish spot has faded and the dark mass has gotten lighter on the other eye. They still look bad, though.

Water parameters:

ammonia - 0 ppm

nitrite - 0 ppm

nitrate - 10/20 ppm (couldn't quite tell)

temp - 73 degrees F (the temp is fine, I've got coldwater and cold tolerant fish in the tank)

Tankmates are 5 panda corys, 5 rainbow shiners, and 5 threadfin rainbowfish. I've also got a lot of live plants. Filter is rated for 50 gallons and the tank's been established for about a year and a half.

Is there anything else I should do? I was thinking of treating the tank with Maracyn II since I read that popeye is often caused by gram-negative bacteria. I do not have a quarantine tank ready ATM, so that's not an option. Will Maracyn II affect my plants or bio filtration?

I've never treated popeye before, so any info is appreciated...I really love my little darter and I want to do anything I can for him. This seemed to just spring up out of the blue...