Does slate have an effect on PH

Hi All sorry if i have placed this in the wrong section wasn't sure where to put it.

I have currently noticed the ph in my tank isn't as high as i would ideally liked, because of this i will at some point be changing the substrate i am using to something like crushed coral, however i am just wondering whether slate could be a cause of a lower ph.

i have a ph straight from the tap of about 7.5 and my tanks are now only just higher than this.

Any help on this would be appreciated.

I've not known slate to lower PH

Ah ok thank you Something has seemed to decrease the ph in both my tanks but i can't figure out what, i will be buying some sand stone or something for the minute to buffer up a tank while i put fresh substrate in to another fishless tank and then do the opposite once it is all stable.

I have never know slate lower ph either, have you added or removed anything else from the tanks?

No i haven't added or removed anything in the past month.
this is what has got me some what confussed i must be honest i haven't done a ph test for the past month and when i last checked all were around the 8 mark and i was happy with that.

I will be changing substrates very soon but one take at a time or i will go out and get some new types of rock to buffer it up that way.