trying to figure out ph issues

I've never ph'd my water before, which does go through a filter. I thought I'd start, cause someone said I should be. Its nuetral 7. When I mix my nutes and test it, it is between 6-6.5. Why when I test my sunshine#4 after feeding, it reads like 7.5? All I'm using for testing is a probe type of tester for the medium, and the little drop solution added to a small sample of water methods.

try testing your run off.let it stand for 10-15 minutes for accurate read.
i ph my soup at 6.0-6.3 in my sunshine #4.

After nutes I PH my distilled water to 6.5 haven't had any issues thus far.

Puff.