Update: pet store test confirmed my results. Different test types and
brands... I must admit that the readings are correct.

I definitely worry about contamination from something. We're usually
very careful, though. I'll have to look into things that might happen
while I'm out, as you suggest.

Considering that I just did a 20-25% change the night before, which I
think is considered "rather large" for tropical tanks, would another
water change be insurance or just stress?

Definitely I'll put in some fresh carbon, something I hadn't thought of.
And I can get a second bubbler in there as well; I don't think there's
any problem with the aeration but under any stress, the more aeration
the better.

In further news, the surviving Bala and the Tiger Barbs seem fine. I may
never figure it out, but I hope I find out if it's preventable. I will
need to put more Balas in there once I know it's safe, won't I? We got
five when we started because all sources I checked say they're social
fish who do best in company.

*sigh*
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Only know that there is no spork.