Have you tested the tank parameters as that is the most important one?
The tank looks good. Have you removed all the rock? If you have and the parameters (Ph) are going down with the water changes then you have probably sorted that part of the problem. If not I would try removing all the rock at this stage (you can always put it back later?) I left my rock a week in the test water before testing so you may be being too impatient, test again in another 4 days as it won't instantly rise it will take time to slowly increase the Parameters (Ph) I expect?
Some occupants were probably weak/sick by the stage you started 'messing' with the parameters etc, and changing the parameters may have just been too much or too late for some, if they were already weak/sick?
I did the tests tonight, and URGH!
Results were useless... I left gravel and the rocks in water for 3 days, and showed the tiniest change (if at all...). I also tried the vinegar test with white vinegar, but also had no reaction.
In the meantime I have lost the rocket killie that was originally sick, and another 1 or 2 😭. HELPPPPPPPPPPPPP!
Should I try with hyrdochloric (or muriatic or even citric) acid just to be sure? And would it have to be pure?
Upside-down strip comparison (srry)
My interpretations: (note- the gravel was in much less water, water barely went over the gravel, but was in for less time (12 hours))
The rocks and gravel
And my problematic tank in all its bad blurry phone photo glory:
I won't hold my breath that it stops there either??????????????
I would probably combine the culls in one tank if you can get the parameters the top end of caridina and the 2 types don't cross breed so don't need to be separated.