yeah you can use genesis with r/o water and hard water, i didnt wanna advise using them though as i hadnt seen any results unlike the gh nutrients their are more people using them in hydro with soft water
if i set my ph to where i want it it stays bang on the button, you do have choices though i just dont think canna nutrients was the best option for your setup but im not saying it wont produce results it could be more hassle than its worth chasing the ph
like i said their are loads of hydro growers using reverse osmosis water but you dont see hardly any of them using canna aqua purely because its designed for hard water
cheers