You can try Eco-Complete African Cichlid Substrate.
http://www.drsfostersmith.com/Produc...&N=2004+113555
It maintains higher water pH without constantly adding pH Up (bad idea anyway, because pH Up just spikes pH, stressing the fish).
Do you have lots of plants? Do you have
driftwood? Are you forgetting to vacuum the gravel?
If you answered yes to any of these, they could be dropping your pH, as they all can produce acids.
Out of curiosity, how many koi do you have in what size tank? I'm with you in being one of the few who keep koi indoors. (well, one koi, at least

)