No idea why but the pH drops by itself after about 48 hours - I don't
have to do a thing. I'm trying to put as few chemicals in the water
as I can.

And I'm fairly sure these are hygrophila, because I had wanted to avoid
that plant after reading it's considered an invasive weed in most
areas--then saw some healthy, unlabeled plants at the local Petco so
bought them. Found out later by google image search exactly what they
were. They seem to make a good canary, though.

The leaves are slightly different on one but the second is definitely
hygrophila polysperma. I strongly suspect the first is hygrophila, but
I don't know exactly which type.

I'll try to find Plantabs, thank you!
Jamie