pH Differences tap to tank

[SPACKlick]

I've just changed my CO2 injection from diffuser to reactor and so am monitoring pH pretty closely. But I'm a little confused about a result I got.

In my tank in the morning before the CO2 comes on the pH is around 7.6. Offgassed (left for 36 hours) it rises to about 8.0. At peak injection it drops to about 6.6 although I'm trying to dial that back to 6.8 because I'm not pushing for massive plant growth.

I just tested my tap water and out the tap it's 6.6 leaning to 6.4. Two and a half years ago when I started this tank the tap was 7.5. I'm going to leave some tap water overnight to offgas to see what it will look like at that point.

Is it usual for tank pH to be so much higher than tap pH? I have inert substrate, 1 piece of lava rock, lots of bogwood, plants and add a small amount of cuttlebone on occasion for the shrimps and snails. I can't think what would be rasing the pH in the tank above tap water so.

[plankton]

How soft is the water now?
If it's been changed to an acidic source it may interact with things in the tank to produce a higher ph, but it would normally make it harder to do that anyway.

[SPACKlick]

Tank: KH = 3 (3.5 maybe) | GH = 9
Tap : KH = 4 | GH = 8

[Andy7]

Cuttlebone has calcium carbonate in it which raise the KH. I am just wondering, but maybe it is the carbonate which is affecting pH? Can't think of anything else it might be.

[plankton]

Cuttlebone won't make any noticeable difference, certainly not that much of a change in ph from 6.5 to 7.5, that's a change of around 1000% if my brain has put all the numbers in the right order, unless there's a whole crushed cuttlebone....then it may make a small difference.
The only other thing I can think of is that the filter "munchers" are working overtime, but that would normally make the ph crash like it does in cycles.

[Andy7]

Yeah, 1,000% is rather a lot. I would have said that, but my head can't count that high.

[Andys temperate tank]

It happened to me once.

My water is similar to yours. It was my "inert" substrate that was doing it. I'm not saying it is the cause of yours but it might be worth having 2 containers of tap water left for a while but put a hand full of your sand in one.

[SPACKlick]

OK, having left some tap water to offgas for about 20 hours its pH raised to 7.9 ish. So I think the local water has just changed somewhat and there's something in there bringing the pH down as it comes out of the tap.

This does bode some concern for water changes because I usually hose water straight into the tank for a big change...

[plankton]

That makes sense now, they must be pumping CO2 or similar to bring the ph down to stop corrosion in the main pipes (although it flows so quickly through you'd think it didn't need to go that low....??).

[fr499y]

bit of extra co2 in the tank wont hurt