RCS colony death
I had RCS for a few years and they have been thriving. I had about 120 shrimp in my 10g (let's call it tank#1). Obviously, that tank was cycled.
A couple months ago, I replaced the gravel substrate with Seachem Flourite and added Flourish root tabs. For some time, everything was fine, but then I started finding a couple of dead shrimp every day.
As of now, the colony is down to 6 shrimp. Initially, I moved them to a 3g tank where a few more shrimp died and then shrimp started recovering. Then I moved them into another cycled 10 g (let's call it tank#2). PH 7.2-7.8, Am-0, Nitrites-0. Nitrates-5, TDS 150 (tap is 99 after adding Prime it's 109), GH3, KH4. The substrate is TopFin aquarium gravel (looks more like big grain sand). The shrimp is dying again. I am hoping to move them tomorrow to the 3g tank again with nothing but moss and gravel. The filter from my current 10g RCS tank#2 will go with the shrimp because it is cycled. I will rinse the sponge in a de-chlorinated water. I use Prime for that.
I don't understand what is killing them. The very first time, when the shrimp just started dying in the tank#1, I tried to move all the shrimp, but missed 4 babies. They are adult shrimp now. Somehow, they survived in that tank#1. But about a week ago, I found one shrimp dead. I decided to move the remaining three into another 10g (tank#2) and today found another dead berried RCS. I am lost. The water looks fine. Is it possible that it's either low GH or Flourish root tabs leaking into the water and killing the shrimp? What can I do to save the last 6?