obviously your tap water has a high pH which is why the tank water is high ... there is not much you can do if that is your tap water ... and i have read on different websites that frogs can tolerate up to 7.6 to 7.8 pH which is only 0.2 - 0.4 from 8.0 pH ... and the frogs are not trying to get out ... they adjust to the pH ... i don't think it is anything in your tank, it is just your tap water ... i too would not use any chemicals to adjust the pH but i might add a big old piece of boiled driftwood ... that would naturally lower the pH a bit ... and not using any chemicals, i am hoping you do not mean you do not use water dechlorinator ... water these days has chlorine and chloramine and leaving the water to sit out will evaporate the chlorine but not the chloramine which is just as detrimental to the fish as chlorine ... that might be a problem in your tank ... i also have a frog tank and the only difference is i have sand not gravel (not that i think that has anything to do with it) so i can not see what you have in the tank increasing the pH ... i would try a piece of aquarium driftwood (boil well so it does not discolour your water) and see if that helps at all ... my friend has a frog tank as well, 2 in a 6 gallon, and she has the opposite problem, her pH is too low ... the frogs adjusted and she has had them in low pH water for over a year ...
What's making you pH rebound is just it's natural buffering capacity--it's what buffers in the water do. With water sources that have a very high buffering capacity, quite a bit of dilution is needed to get the water down where it needs to be--adding a little purified water won't do much, you'd need to take out a lot more than that. Letting the water sit does nothing to the pH of the water either.
Just so you know, a pH of 8.5 is still ok for fish and other aquatic life. I'd recommend leaving your pH alone, as screwing with it can actually be more dangerous. Rapid changes in pH are more stressful to fish and your frogs than just a high one. Your frogs swimming around and trying to climb out is not a sign that they don't like the pH--it's just what frogs do sometimes.
If you really want to soften the water, you can do partial water changes with some RO water until the tank is at the desired pH. Do this a little at a time, like I said, rapid changes are a bad thing. I'd recommend premixing your replacement water for future partial water changes to the matching pH, so that you don't have to worry about parameter changes.
all you have to do is go to your local pet shop and by a ph balancer, works every time.
PH level is high maybe due to oxygen level in the water, rock or substrates available, constant water changes, and other compounds especially bacteria that build up too often. Hope this help.
DOn't worry about your ph thats nothing you should be concerned about.. but if your frogs are affected it by it then go to the pet store and buy a chemical like API ph up or API ph down
But really your just gonna affect the fish more if you change it and they're not affected by it :/