Why Does My PH Change?

Back from a weekend away, and I've just tested my water and the pH is really high!  7.8 kind of high.

I live in a very soft water area (oldham, near Manchester).  I tested the pH as it comes out of the tap, and that is 6.8. So why on earth is the pH in my tank 7.8?

I do have a shell in there, which I guess could theoretically leach some carbonate into the water, but surely not enough to have such a big effect?

I do put some of the 'tap safe' stuff in when I do a water change.  Could it be that?

Any idea what could cause it, and should I be worried and should I do anything about it?

Ahhh.
Yes, the 6.8 came from a very freshly run sample.  I will put some to stand now, and test it tomorrow and let you know.

Thank you.

What should you do about it - nothing!

Messing with pH is not to be undertaken lightly. It can go disastrously wrong and leave you with a tank of dead fish.

Fish can cope with pH outside their 'correct' range but they cope less well with hardness outside their 'correct' range. For your water that means soft acid water fish will cope better than hard alkaline water fish. All pH measures is the amount of hydrogen in your water. With a higher pH you have less than people with low pH (it's an upside down measure). Unless carbonate hardness is specified, hardness means GH and that is just a  measure of the amount of calcium and magnesium in your water. It is quite possible to have low levels of calcium and magnesium and also less hydrogen in the water.
I have GH 6 and pH 7.5. I can keep soft acid water fish quite well. I have green neon tetras and Sundadanios, both of which are supposed to need water below pH 6.5. But these two species also need soft water which I have.

Concentrate on fish that like soft water and ignore the pH.