A friend of mine has a 30 gal tank with a few fish (2 gouramis, 3 rosy barb, 2 zebra danios, pleco). I take care of his son, and while I do have my own fish, this question involves his tank, as one of his barbs is very ill and he's unsure what to do about it. I told him I'd ask the forum.
Essentially his largest Rosy Barb, who he has had for a few years, suddenly became ill. He is twitching when he swims, and often lies on his side on the bottom of the tank, just breathing. My friend is not sure what brought this on, but he did say he did a w/c two days ago, and threw in some lettuce for his gourami who is suffering from Dropsy. He wonders if it could have been the w/c, or perhaps the lettuce? He has put veggies in the tank before and Lucky (the Barb) has not had any reactions prior.