Good quality hydro nutes should have a pH buffer. I had this exact problem the first time I loaded up my new hydro system with clay balls. It was about a week before I stabilized the pH but there were 2 problems that I hadn't considered:
-Not rinsing the balls. I used plain tap water just like you did, thinking the DUST would be the big issue.
-Using a dirt fert that was buffered in the high 6 range
My pH would spike up to almost 8 for the first couple days. The plants suffered greatly while I messed with them but there was only one casualty. It was jsut a matter of testing the res morning and evening and adding pH down until it was in the mid to high 5 range. It stabilized eventually. I also switched to canna nutrients, which I now run with just plain tap water. I'm not sure if the intention was for them to be so well suited to that... but they are, and buffer just below 6.