Restocking A Slightly Acidic, Softer Water Planted Tank

The vinegar test is what I've done before.

What else is in your tank?

It would be interesting to see how quickly the hardness increases following a water change...

Are you adding any chemicals to the tank other than dechlorinator?

I'm not adding any fertilisers at the moment. I wanted the tank to settle after all the rearranging so that I could get a baseline from which to work out the correct amounts of any required supplements.

During the period of neglect I topped up evaporation once (about 5l in 170l).

The amount of water in my tank has increased from 170l (measured at initial setup) to 220l (estimated) with all the substrate removal. Does that count?

I am contemplating doing a couple large water changes tomorrow to see if that makes a difference. Think I've run out of time this evening.

If I do go down the route of large water changes, are there any limits so I don't cause my existing fish distress?

Should I do a larger than normal water change before getting at new fish?

I've just wondered if perhaps it is less of a problem than I initially thought.

My hardness in my tank is 107ppm (moderately soft), in comparison with the hardness of my tap water at 54ppm (soft). Both are easily within the preferred range of all my current fish.

It is the pH that might be an issue, but I keep coming back to lack of CO2, as I have no better ideas.

Do I have enough buffering (Kh = 72ppm) to avoid a potential pH crash if I add CO2?

If my tap water is softer than my tank water, would there be much benefit in using RO to top up?

Are you testing your tap water straight after taking it from the tap?
The pH of my tap water changes over a 3 day period. It is 7.0 - 7.2 straight from tap, then increases to around 8.2 over around 3 days. This will also be reflected in your tank, as it is in mine.
I have quite hard water, so don't know what sort of impact the changes would have on your tank. To avoid large(ish) fluctuations in your water parameters smaller/more frequent water changes would be better.

There are also discussions to be had regarding the parameters listed for specific fish, and whether they were wild caught or commercially bred (in different water conditions), etc.
Unfortunately it can be a complex discussion, and I've had a couple of glasses of wine tonight, so I may have to pick this one up again tomorrow.

Just saw your other post.
My previous discussions with Sue and others concluded with us thinking that perhaps the water company added CO2 a source to ensure that pH was neutral at the tap. The would gas off over a couple of days, revealing the true pH of the water. I would have expected that dechlorinated water from the previous week would have shown the "true" pH, which may still be lower than your tank pH.

I would suggest you do a large water change @Helen (and in answer to your previous question this means no more than 50%) then give the tank a few days to settle and see what your test results are. If they have changed since your previous results, give it another week and test again.  Again, see how they have changed. We want to get to a point where we understand a) what's causing the hardness to increase in your tank, and b) what the stable hardness level is likely to be once things settle down in the tank.  The above plan should give us a good feel for both. 

I changed 105l this morning. 
I can't see me ever changing more than 50% of the tank water unless under dire emergency. It was quite hard work.

While I was doing it, I got to thinking about all our recent discussions. Over the Christmas break, we've had a lot of children in the house. My kids know not to touch my fish tank, but I realised that young children seem rather fascinated with the magnets that hold my Seneye in place. And so it gradually moved up the tank, as far as I could get it. I have now moved it back to what I think is the most sensible location - in the middle (horizontally and vertically) of the short side of the tank (furthest from the filter).

I know position in the tank will affect the temperature readings, but does it affect any of the other readings? Also the water samples I did the tests on came from the top few cms of the tank water.

And then I got to thinking about what you had said @Littlefish  about the water companies adding CO2 to reduce pH to neutral. If my problem is lack of CO2, then any additional CO2 can only be good for my tank. But also, if I am concerned about pH swings from large water changes, then I really need to know the pH of the water when it goes into my tank (rather than when it has had time to off gas).