WOW I am surprised you have that much polution since you mentioned you are in the middle of nowhere, that's aweful and sorry about that! I think your water choices are fine, I just don't understand what happened to your poor frogs. Sadly without a necropsy you may never know what happened, but I hope if you decide to get any more frogs they do better than these.
Maybe you can try another breed that is hardier, like Gray Tree Frogs. I just love mine and from what I understand they are very easy and hardy tree frogs, not to mention very entertaining and wonderful tame pets. I have 13 now and a entire pool full of tadpoles, I'd love to send them, but I am still waiting to hear from the Pa Fish and Game Comission on that issue. If you wre closer I'd just drive them over

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Are you in Canada or US? I will check with the Pa F & G Comission about crossing states and country borders too.
I know that there are several websites that are linked and or reccomended maybe you can try ordering a new frog/frogs from there. Best of luck.