I read through this thread and saw no mention of pH fluctuations. The only mention of pH has it at 7.5. A KH of 4 degrees is not high certainly, but is going to provide some pH buffering. You should do a series of pH tests over a few days to ascertain just what is or isn't occurring. You want the pH of the source water (I assume RO at present), and the tank. Test the tank water at the same time each day you test; there is a normal diurnal fluctuation. You could test twice daily, once in the early morning (meaning, when the tank light has come on), and once in the evening (when tank light is about to go off). This will give you the diurnal fluctuation. Done over a few days you will see how consistent it is.
I would expect from the data that your pH will be acidic (below 7). The limestone rock would raise it, which may explain the 7.5 reading. The above tests will hopefully sort this out.
Ok so update time. Big 60% water change on wednesday. KH is at 5.
pH measurements have been consistent, but confusing.
So in the morning my high range pH test (API master test kit) has been 7.4. No higher. Its bang on the same colour.
Just after the lights have gone off at night it is nearer to the 7.8 colour.
That isnt the confusing bit. The confusing bit is that the standard API pH test has never moved from the 7.6 colour blue - even for the test showing 7.4 on high range. I know if the pH is higher than 7.6 then it will still be the same colour. Same way if its lower than 7.4 on the high range then itll stay the same colour.
Testing the source water pH shows that the test does work and isnt just a dodgy bottle.
Im wondering why my pH is still higher than the source water? Ive done 3 50% changes since saturday last.
I have 3 rocks in my tank which look like normal rock... Im confident theyre inert - im friends with the manager at my LFS and hes insistent they only stock rockware that wont react with the water. I trust him on this - he gives good advice. I dont trust his staff one bit but i really trust him
Other things in tank: substrate is JBL sansibar red sand. Its marketed as not affecting pH or hardness
This cave ornament thing: Rosewood Cave Complex Aquarium Ornament, Small
https://www.amazon.co.uk/dp/B00ZRX3PL6/?tag=
A lump of driftwood.
Its currently not as heavily planted as id like. 2 ozelot swords, 2 anubias nanas, a single bacopa which has been pruned and replanted 7 times... So its more like a bunch of bacopa... Some pretty large water fern (dont know the name) and just under 1 third of the surface is covered by amazon frogbit.
My only other thought of the higher pH could be if theres phosphates? I had to replace my fiter 6 weeks ago and one of the foam pads is constructed from a pad in my old filter which wasnt very clean and i havent cleaned the filter yet since running because i was paranoid about my tank cycling.
I rinse the pads every few weeks.
As to the fish health:
Still active and feeding and are all their usual selves. Great colour on all them. No whitespot. Ive only seen the one fish flash but that was an isolated occasion/potentially knocking algae off a leaf (nibble nibble nibble flash nibble nibble etc. All on the same leaf)
The one fish that flashes still flashes after i feed him flake! Ive tested other foods. Had 3 days with no flashing. Feed a tiny pinch of flake and he flashes.
As flake is relatively inexpensive and the stuff i use is 9 months old im going to try some new stuff. Any recommendations?
At this point im not concerned about parasites/ich. I think something in the water (probably whats raising the pH) has irritated that fish's gills. And as flake breals up into such small bits im wondering if some gets in his gills and annoys him?
As to other fish flashing previously - ive only ever seen them do it AFTER the other fish has. My cherry barbs imitate every thing. They copy the cories when they rustle for food. If one cory darts to the surface for air then all of the cherry barbs copy the cory - they swim the exact same line, dart at exactly the same speed etc. They never do it unless a cpry has just done it... So i wonder if the others had imitated the flashing?
Or it could be a territorial thing? The one who flashes most is 100% the top dog. Beautiful deep red colour, by far the biggest (a 2 inch male cherry barb is quite the spectacle), he flairs his fins a lot at the other males too... So maybe hes doing it in a similar way cichlids do?
I'll post on here on saturday and then subsequent weeks so that if anybody else has something similar then they can use it as a guide 
).
