Could be ferts but I would worry much more about your ph than the ferts.....ph being abnormal could certainly do that to your plant as well. I'm afraid I don't use either the tab nutes or foxfarm so can't give you a lot of info on either one.

Going with plain water for now was a very good idea.....good work there. Get yourself a new ph tester asap (even the cheap liquid ones from the pet store will work ok for you in a pinch) and get your ph straightened out. If you post the results I'll keep an eye on this thread and hopefully that will solve your issue.

Good luck.