You mentioned "wild guppies". Do you happen to know the species? I have gambusia, and they can be nippy and aggressive - which can cause stress for the other fish. Stressed fish are more prone to getting sick. Stress usually doesn't affect water quality, but poor water quality can cause stress.
Also, I'd recommend a master test kit over test strips (which is a recommendation I've seen from several members). It will give you a more accurate reading.
As to vanishing fish, I've had that problem too. Snails will make quick work of a dead fish, as will other fish and scavengers. Snails are not necessarily bad - they may not be effective on algae, but they do clean up uneaten food.
The most common recommendations I see posted everywhere on this site are regular testing and weekly water changes (usually 25%).
Someone mentioned kids and cats, and I have to comment on this. I was out running errands with a friend who left her 13 year old daughter and 7 year old son alone in my house. While the daughter *should have* known better, she's a lazy snotty drama queen who resents watching her little brother, so she parked her substantial butt in front of the TV and ignored her little brother - who ran rampant through the house. I got home to find he'd been in every one of my fish tanks, trying to play with, pet, and hold my fish. (He'd also moved my "cricket" tank - which really houses a scorpion - while trying to open it to play with the crickets. This was in my bedroom, which both children were instructed to stay out of.)
c:
