Very Unwell And Wobbly Tiger Barbs

Umm, I've lost another 2 tiger barbs today. 1 standard and 1 green.   
Things had been ok for a while, and they had a regular substrate syphon and water change yesterday.
Pre-water change parameters were 0, 0, 40.
Tap water today is 0, 0, 40.
Tank water today is 0, 0, 40.
The only difference is the pH, which is slightly higher in the tank (7.8 ) than from the tap (7.4). This happens in all of my tanks, so if anyone can help me with that one as well that would be great. Have tested the hardness of my water and it is 17dH, as always.
The tank has aquabase covered by a layer of gravel. It has live plants, several pieces of mopani wood, some rocks, and the hollow log they've always had.
The female who had her tail bitten off during frisky times is growing it back.
The two tiger barbs lost today did not have any symptoms that I saw yesterday, and didn't have any physical injuries today.





Edit - I have discovered if you type 8 followed by ) it turns into a smiley. So I've put a space between them and the pH now reads 7.8 instead of 7.
Sue

It's been a bit of a day, but the smiley thing made me chuckle. Thanks for the edit Sue.

The tap water was fresh, but I've got some dechlorinated water sitting overnight, and I will run some more tap water and leave it overnight, and test the pH on both.
I think that it probably is a concidence that the recent problems were post-water change. Previously the problems decreased with water changes, and I've been doing regular small water changes twice a week since the problems cropped up. Perhaps the water change just stressed some already poorly fish. I have been keeping quite a close eye on the tank, and didn't notice any wobbly swimming this time.
Whatever it is seems to be happening quickly, or not showing any symptoms apart from the final one.
I've not seen any worms associated with the barbs, and the fish all seem to have been eating and behaving normally.
Is there anything I should be doing for them now?

I can verify that the chlorine being added on any particular day thing is not accurate.  Each sampele of water in the UK which is taken from our taps must meet criteria for bacteriological levels (0) and must have enough chlorine in to ensure that this is the case. Chlorine is therefore added consistently in an automated fashion.  Further to this, between the water treatment works and our homes there are a series of storage tanks which help balance flows between our constantly fluctuating demand for water and the more consistent production at the treatment works. This sometimes requires rechlorination to boost levels again in our supply. I believe this is where this belief comes from, however the chlorine is added where it can be easily mixed i.e. dosed constantly into the pipes entering and or leaving the tank, rather than dumping a load into the tank directly for example.

Hope that makes sense... I'm sorry to have ruled out something else without having a further suggestion.

That said, It strikes me that the choice currently is to either ensure excellent water quality to allow the fish to fight whatever it is by themselves or to treat the water to help them fight it off.  It might be that from this you reach the conclusion that given  that there have been further deaths despite maintaining excellent water quality, it is time to treat.  this is not easy however, as currently the cause is unknown and medications are generally specific. My somewhat overly logical approach here is leading me to think about what ''general" medications might be worth trying OR what else could be done to help either give a little boost to the fish or help kill anything that might be impacting them but is ultimately more susceptible to changes than the more robust organism which is the fish such as adding a little salt, increasing the temperature etc.  To be clear, these arnt suggestions as such but just where my logic takes me.  Does this prompt any thoughts for anyone?

Several years ago, one shop told me that some of his customers had unexplained fish deaths after water changes on a Saturday. One customer phoned the water company and was told that they flushed the pipes with lime on a Saturday. Remember this was third hand information so I always presumed what the water company actually did was use a chemical to flush limescale out of the pipes. Which also doesn't make sense as we have soft water so very little limescale  Unless they did actually flush the pipes with lime 

Whatever it was, they did something on a Saturday.

That's what I thought, but as the pH goes up in all of my tanks I wasn't sure what it could be.
Most tanks have JBL Aquabasis plus as the planting substrate, covered either by aquarium sand, aquarium gravel, turtle pebbles, depending on whether or not I have bottom feeders in the tank. Axolotl tanks only have sand substrate.
All tanks have some bogwood and usually also mopani wood, and all have live plants.
Not all tanks have rocks.
Most tanks have either a fake hollow log thingy, or fake rock cluster/stone bridge thingy, or similar, which are polyresin.
I think I'm going to have to investigate this further. I have an empty tank or two, so much spend some timeover the next few weeks/months slowly setting them up and testing the water to see when the pH starts to change. Will also be getting the vinegar out to test the rocks.
It might be nice to get to the bottom of any many of the issues as I can.