Advanced Nutrients is a pretty reputable company and should not be screwing up your soil chemistry unless there is something else wrong with either your soil or your application method, since we know your water is fine.

A good pH test kit to start off with is only about $7 for the liquid indicator drops, and I very strongly recommend the drops over a cheap tester, since inexpensive pH meters are prone to errors that you might not catch if you don't calibrate the meter and check it in buffer solution each use.

You need to flush out your pots with plain tap water that has been adjusted to the correct pH, and then when you fertilize, make the fertilizer solution and then adjust its pH, too.

The whole General Hydroponics pH test, up, and down kit only costs about $15-$20 US and is everything you need to do this. I'm a professional chemist, and I'm too cheap to buy a 'real' meter, so that's what I use, too!