Thank you both for the advice. Unfortunately, I had a disaster shortly
after I wrote this message. I'd changed the water in the tank today using a
different water source than usual. A few hours later I went by the tank to
find the fish floating. I am afraid that the water source I used had either
soap or detergent of some sort on it, and it killed them quickly. I tried
water changes, to no avail.

I feel pretty lousy, needless to say. Two healthy fish, gone in almost an
instant.

The same thing happened to me a year or so ago. I changed the water without
using dechlorinator [I had been adding it after, my house has a dechlorinator
built in, but it was broken that day]. Killed all 4 of my fish in a matter of
minutes. :'( Sorry for your loss.