Thank you for your quick responses!
We collect rainwater here and I've always drank it without problem, but…new apartment...and a new roomie who prefers filtered water...I've been drinking the filtered water he puts in the fridge but filling Paco's bowl from the sink, so I can't tell if it's the water. The neighbors upstairs let their cats and dogs drink the unfiltered water, and say they don't have a problem; maybe that's because they've been drinking it their whole lives?
I've switched to filling Paco's bowl with filtered water in the meantime, but I guess if something in his system already from the water before, that that won't matter.
I haven't gone to the vet yet because, besides the stool, Paco's been eating, drinking, playing, and behaving totally normal. Everything seems fine except for this one thing. He just went for the 3rd time today...still soft, but not "watery." I still haven't fed him for the day and won't until later tonight when I'll boil the chicken and white rice I picked up. I didn't bother grabbing pumpkin based on you guys' advice.
I'd really like to try to solve the problem at home before I go to the vet, but if the problem doesn't get resolved by the end of tomorrow, I will go. I just haven't tried anything yet and would hate to pay someone to tell me to put him on a boiled chicken and rice diet for a few days when I already plan on doing it...especially with the new apartment taking up most of my finances as it is.
Thank you again, guys. I will let you know how it goes.