Fat Guppy

So I have a guppy in my tank that essentially tries to eat everything in sight, a whole lot more than the rest of them.  Am a bit stressed out over the aquarium today...the guppies (2 of them) were pooping white and it's suppose to be brown because of their food.  Am not sure if I am underfeeding or overfeeding them and it's caused that.  I read that it could possibly be internal parasites...a whole myriad of things.  Either way, been feeling rather panicky about the whole thing.  I also did a major water change today because I discovered that the PH levels were off and they were still off an hour after the water change.  Anyway, I don't know what else to do.  I fed them a pea tonight and am hoping that by not feeding them tomorrow that it clears out their systems.  Feeling a tad disheartened and my head hurts from reading so many various articles on the web.