Female swordtail is acting weird

We bought 5 females.  One of them gave birth immediately after being transported
here.  Probably stress.  I saw two fry and that lasted a week or two then they
disappeared.  The mother fish died the first night in the tank.  The others
limped along, another died a few days later, then another.  We have two left.
One looks okay, the other is smaller and swimming fast, short distances and at
the end of the thrust, her tail dips.  She's eating, no discoloration or
abnormal looking ulcers or anything.  No bloating.  Just this swimming rapid
burst and tail dipping.  It looks like she loses her balance or something.
She's been doing this for about 5 days.  Eating.

We had some high pH and have been trying to adjust it using driftwood and
rainwater changes.  We got it down from 8.5 to 7.8 over a three week period.
The tank is kept at about 78.

Any ideas?