Thanks for all the replies and advice. It's really great to have such a supportive community that can provide advice, especially when you're a bit of a worry-wort like me.
I didn't use any medication. I am extremely reluctant to put anything like that in my tank, especially when I'm not 100% sure what the problem is. Instead, I ended up removing 2 of the platies, and the remaining 1 "got better" almost immediately. I put "got better" in quotes because I'm starting to think they were never sick to begin with.... but that's the subject of a new discussion that I'm starting under the Fish section....
55g: Finnex Planted+ and MonsterRay, Pressurized CO2 w/ Atomic inline diffuser, Eheim 2217, Eco-Complete, EI ferts. Lemon, Glowlight, and Black Neon Tetras; Harlequin Rasboras; Otos. Staurogyne repens, Hygrophila corymbosa, Ludwigia repens, Ludwigia glandulosa, Red Melon Sword, Wisteria, Water Sprite.
Last edited by Smeagol; 04-20-2013 at 02:58 AM.
Reason: more detail