Here is the ideal situation...
you would use a test kit for ammonia to monitor to the levels, this simple procedure alone will tell you exactly when water exchange is need to be performed.
Otherwise, you would do a 25 percent water exchange every week when you clean your filter. You should only do a full water exchange when the tank needs to be completely broken down and cleaned.
Simply put, water exchanges ultimately stress the animals, if it were not for ammonia, you shouldn't have to do them at all!
Also, just because it is store bought water does not mean that it is safe. Check the label, even Springwater will have a statement to the effect, bottled from a municipal source, that means it is chlorinated and no better than your own water. It is cheaper and environmentally safer to treat your own tap water, assuming it is drinkable at all.
I have enclosed a link to lessons on water quality, read through it all, 75 percent of that web site applies to you and keeping of your frogs.
click here for the link
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...the oldest task in human history: to live on a piece of land without spoiling it."
Aldo Leopold (1938)
"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)