What is causing my high pH in my tank?

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 ...