Changing PH In Tanks

Towards the end of my thread about my unwell tigers barbs I noted that the pH in my tanks was higher than that from the tap. I left dechlorinated tap water and plain tap water overnight and tested the ph, which again wasn't as high as in the tanks. In an effort to investigate this issue, and as I had a spare 70L tank hanging around, I decided to add dechlorinated water in the tank, plus a filter, leave it running and test the water. The idea being that I would add various items from my other tanks to work out what was causing the pH change.
The tank was set up last Friday. There is nothing in it apart from the water and filter, and the lights go on every evening, the same as with the other tanks.
Dechlorinated water going into the tank was 7.4.
I've just tested the water in the tank and it is reading 8.2 using the API master test kit.
 
Any ideas?

I use Tetra Aquasafe as my dechlorinator. I know it's got lots of other things in it, but it's one of the ones suitable for axolotls, so I've tended to use it for everything rather than buy different types.
I'll set up the tap water/dechlorinator experiment and update you on the results.

Both are 7.4 at the moment.

I appreciate your help Sue.
I don't want to be killing stressed fish by shocking them with a change of pH when doing water changes.

Does oxygenation impact pH? Just wondering as some of my tanks have substantial water movement and/or an airstone, and the one I've just tested (dechlorinated water & filter) has a lot of surface water movement.

Great information & explanation @Sue and it also explains why the tank with no plants is showing the highest pH I've seen in any of my tanks, and my plant tanks (for just storing plants whilst waiting for new tanks to arrive) have lower pH. I wouldn't say that any of my tanks are particularly heavily planted, perhaps that's something I might have to consider.
I'll be testing the rest of my tanks tomorrow. 

I realised after I'd turned my laptop off for the night that I had omitted to say how carbon dioxide gets into the water in the first place - it's by the fish etc respiring. Animals just make CO₂, but plants both make it and remove it.

And with a heavily planted tank, the time of day the pH is tested makes a difference. In this case it will always be highest when the tank lights are on and lowest when both tank and room lights are off (if you feel like sneaking in for a water sample in a dark room  )

Another point I didn't mention is that if you add CO₂ to the tank, that will alter the pH as well. Lightly planted tanks and those with low tech plants don't need added CO₂ as the fish provide all the plants need. It is heavily planted high tech tanks that need additional CO₂.

Just checked some pH levels.
8.4 in the tank with just dechlorinated water & running a filter, which tested at 8.2 3 days ago.
8.2 for the glass of dechlorinated water sitting there for 48 hours
8.4 for the glass of tap water @ 48 hours
8.4 for one of the plant storade tanks - running with filter, plants in pots, lights have been on for approx. 6 hours.
8.0 for the puffer tank, lights have been on for approx. 4 hours.
I have turned the light off in the plant storage tank and covered it with towels, and will take another sample to test later.
I have no idea what is happening.   

I am going to be putting more planys in the dwarf puffer tank over the next few weeks, and some more bits of wood, etc. for them to explore, so I will probably have to go down the route of adding liquid CO2 at some point to help. I'll be interested to see what happens to the pH then.

I've also just read some information on banana leaves, mulberry leaves and catappa leaves being used in tanks. I'd heard of catappa leaves, but not the others (I mean, I was aware that the plants had leaves  , but not that they were dried and used in tanks). They say that they can lower pH, and that the mulberry leaves are a good source of foor for shrimp - might be useful for info for @pandaman . They say the banana leaves have similar properties to the catappa, so I was wondering if it would be worth getting some (banana or catappa) for the tanks. If it does change the pH with it also impact the hardness of my water? If so, as long as it's not a big change, will this matter?
Basically, is there anything that can go wrong if I add any of these leaves to the tanks, as long as I do it slowly and keep an eye on the pH? Don't want to have an epic pH crash as the results don't bear thinking about.
Any advice welcome.
Thanks. 

I've also just read some information on banana leaves, mulberry leaves and catappa leaves being used in tanks. I'd heard of catappa leaves, but not the others (I mean, I was aware that the plants had leaves  , but not that they were dried and used in tanks).

I've used catappa leaves for quite a while now, and, following the comment about banana leaves from pandaman recently, also bought some of them (they came all the way from China). As you've probably gathered, I'm quite cautious about adding anything for fear of anything happening to my fish. I've not found that either of them have affected my PH or GH/KH at all [edited to add - and, following Sue's comment above which overlapped with me posting mine, it seems that this would be expected to happen more in a soft-water tank such as mine]. The only adverse effect which did occur was when I added the water that I'd boiled some catappa leaves in to the tank after letting it cool, to give the tank a more tannin-stained effect - the fish started behaving oddly and RichardW suggested that it may have resulted in an overdose effect which certainly seemed to have been the case. If you fancy trying them out before committing to buying them, and are happy to P.M. me your address, I'll pop a sample of each off to you in the mail. Incidentally, I see that it's also possible to buy alder cones (Dennerle make them, I think), but hadn't read of mulberry leaves - very interesting.

There is some fantastic information here: http://www.practicalfishkeeping.co.uk/features/articles/2016/10/28/free-tank-decor about tannins, coloration and affect on pH from different leaves. 

Edit: scroll to the bottom of the page to see the charts

Freshly run tap water is 7.4 and earlier you said a glass of tap water had pH 8.4 after standing 48 hours. Are these both correct or is one a typo?

If they are correct, it indicates that the water company adds something (possibly just carbon dioxide) to acidify the water in the pipes. When the water stands the excess carbon dioxide gasses out like gas from a glass of lemonade and the pH rises.

Last time I tested it, my tap water pH went up 0.2 on standing.