OK, here are the parameters:
New, unconditioned tap:
Water in which a betta has lived for around 42 hours. This is tap water that has been conditioned with Seachem Prime. 8 drops for 1 gallon.
These were done using test strips.
I don't understand why there is any ammonia since that is more than the usual amount of Prime and it has been (slightly) less than 48 hours, so it should still be in effect.
It's worth noting that the bettas are doing fine in this water, slightly lethargic, but nowhere near as bad as it has been. The redness disappeared after this particular water change.
The next time I see the redness, I test that water. I don't know why I didn't do that earlier. I think it was just the panic at seeing it happen.