It certainly has a weird colouration.
At the moment, all I can suggest is removing the cholla wood and alder cones. Just as a precaution.
I would also do a large water change, just in case there are any toxins that can be removed. 80 - 90% water change if you can manage it.
Looking at your great set up and from what we hear so far it is almost certainly something specific to this tank so removing anything that is different from the others is a good way to start. If it were that something was poisoning the shrimps by releasing something (pesticide etc) into the water you would expect that to gradually get worse as the poison level gets higher over time, and that sounds that that is what has happened! By removing the source (though we don't know what that is at the moment) there may still be some that the substrate has absorbed of course?
Were the shrimp deaths all in one area of the tank (clutching at straws) and was that near the bamboo?
I would remove anything that is different from the other tanks as a tester.
Did you see a cycle on the tanks, I have never seen one on my tanks with a PH as low as yours/mine (PH 5 - 5.5)? I'm just interested as with a low PH (below 6) ammonia is the less toxic ammonium from my research. I doubt this is relevant to your problem though!
ps. As I say I had the same problem and was down to 1 red bolt (I think that died in our heatwave in the past week) just before lockdown but I have put fish in that tank for now, 10 chilli rasbora, 1 killi. I will try shrimp again when (IF) the post gets back to normal. I never did get to the bottom of what the problem was in my tank though in all honesty! Your tank looks same as mine, as does the light.
I would mix the new RO water to TDS 160 (you don't need to do the GH test everytime as the product you are using will mean that the GH is fine)! The KH alixir I use does add some KH but as I would be aiming for tank TDS of 180 (roughly, it doesn't need to be spot on, 10 either way is fine) ultimately then the extra TDS that the KH adds should be covered!
From your figures above you can see that the GH+ at the level you used creates 1 GH at TDS of 20, thats usual!
Ok thanks. Sorry yeah the TDS is 129. What TDS should I be aiming for? From what I understand when i remineralise my ro water, I should aim for a particular GH then test for the TDS that correlates to, which is then the TDS i should aim for in the future. In my case, following the dosage for my Bee Shrimp GH+, my RO water ended up with a GH of 7 and TDS of 140. So should I aim for 140? A TDS of 140 is still a bit on the low side I think but if were to aim for lets say 170, would that mean GH would also have to go even higher? or will the addition of KH make up for that extra TDS?
I hope that makes sense haha just trying to get my head around it!
Top up should always be pure RO water omly for the reason you mentioned.
Can you retest TDS? It is probably still too low but hopefully going in the right direction?
If the shrimp are having moulting issues then it may have damaged one of the eyes, I've not heard of one losing an eye though?