The first thing is to move the sick frog into a hospital tank (bare minimums, hides, paper towel, raise the temps and keep it covered).
Then as far as the current setup, I would highly suggest removing the salamander. I would never recommend keeping two different species together, I have not heard of any people being able to keep different ones together for the normal lifespans of the animals. Each have their own bacterias, parasites, and pathogens that can be fine with their own but deadly with others. It usually takes time, it mixes in the habitat, into the water sources etc. Besides the fact that there can be stress from having to deal with others blocking access to water, hunting etc.
As far as the water I would suggest using a spring water if that's at all feasible. If not I would make sure at the very least you use a product to remove the chlorine, chloramine etc. You never know what you're getting with well waters.
After doing some research this also may help you (from the amphibiancare.com site) they are actually native to North America and don't need a humid habitat. You could drop the humidity down to around 50%. (Besides the higher humididty breeds all kinds of stuff)
The temps day should be in the 68-78 range so I would look into adding a supplemental heat source such as a heat lamp, or a higher wattage light, or in your case, with all the water you could add an underwater heater; that is what I have and it works beautifully for our colder months.