Transferring shrimp to a new tank

[Punchy]

I have cherry shrimp in a 30L tank which are thriving but numbers are getting high so I got a new 60L tank for them.
Its been cycling for 2 weeks now and all parameters are identical to the 30L tank - temp, ph, kh, gh, nitrate, nitrite. I've also moved a few plants between tanks to help.
I added 4 to the new tank today before transferring them all and all 4 died within a couple of hours.
Originally, most of the shrimp were added to the small tank after only about a week of cycling and I only had a couple die so i thought 2 weeks with the new tank would be plenty. I've also used Seachem Stability to help provide beneficial bacteria.
I have no idea what I am missing or doing wrong... any ideas?
Thanks

 

[Autumnsky]

Did you use dechlorinator?

 

[Punchy]

Yep, I use RO water from my rainfall tank so there should be no chlorine but i still use Seachem Prime anyway to be safe. I added another 3 yesterday and 2 are still alive so hopefully the problem has somewhat resolved itself, or these ones were just more hardy.

 

[Autumnsky]

If you use rain from a roof type system, some collections of water could be more polluted.

Say water from rain after a longer period without rain allows more soot, bad air particles, which could be worse after fires, or if you are in or closer to industrial / manufacturers, or city areas vs. a rural area far from commercial operations causing air pollution. In a class I had, there was an alert about rain water becoming unsafe because of what can be concentrated on the roof.