weird problem

[plspls]

i have a weird problem apart from me going mad ,any way on a serious note my ph used to be 7.60 at the beginning of the year now my ph is 6.85 i thought it was the meter and prob so bought new jbl co2/ph meter only to discover that my dennerle was giving the same readings even done a api drop test kit that is giving same readings on colour chart I'm wondering if my substrate would have anything to do with it .if i go onto one of the tanks that has corel sand in the reading is 7.30 in my main 6x2x2 which i have been setting up for plants and discus and angels has the dry clay substrate in it and root wood i even turned off all pumps run an extension to different socket in house turned of all lights still made no difference ,so I'm at my wits end as i was hoping to start pumping co2 and having a high tech tank .by way the ph in the area we live straight out of tap is 7.60 all help would be appreciated from mad english man in northern ireland kh is 8

 

[rebel]

What's the kH in the problem tank? You may have old tank syndrome as it's called....