With the fish that's left, is it eating ? If so, try feeding it a shelled split pea, frozen peas work well, just peal the shell off of it, and cut it into small enough pieces the platty can eat, if it's a swim bladder issue, this should help, it should also help increase it's apatite.
Beyond that, if your PH is significantly different than what the fish are use to, it could be putting them into shock, best bet would be to test the water they come in if you purchase any more, and compare it to your own. Do not try to match your water to theirs, rather slowly add some of your tank water to their water over the course of an hour or three. If you can add an airstone to their baggy while they adjust to your water temp \ conditions. Fish can adapt easily to most water conditions, you just have to let them do it gradually if there's a large difference.
When you can, you should look into upgrading your testing from strips to API's Master Freshwater Test Kit, they go for about $20, are much easier to read, and much more accurate.
One other thing you may want to do also is setup a Quarantine tank, changing environments can be stressful, being chased by a net, being put into different water, new sights, new sounds etc. If you salt your quarantine tank it'll help your fish adapt, by increasing their slime coat, and it also will help de-stress them. You'll also have a buffer for your tanks to help weed out diseases or parasites from entering your main tank.
Lastly, if you have a Local Fish Store, some mom and pop pet place, you'd likely be able to buy healthier fish there than from wal-mart, however if you don't have a better choice, then you'll probably always have a casualty or two when buying fish from there.
