Being the devils advocate.. To publish names without proof is not good..
It could have been the plants, no doubt about that, but then it could have been anything else. Although it points to the plants, as it seems the only constant on the two tanks.
Also, you need to share some of that blame; with such an expensive collection of shrimp, and living in London, with ample access to a shops that sell Tropica, and also knowing that Asian plants more often than not have pesticides sprayed on their plants, why would you buy or not have enough care with any plant that you would use on a shrimp tank?
Many people here have lost shrimp or fish due to pesticides, many of the leading experts on the field advocate even using only in vitro plants on shrimp tanks.
I completely understand that we look for cheap plants, and instead of buying safely, and actually supporting the local trade, we buy cheap, and then risk loosing a lot of money, like you did.
I know who the seller is. I buy often from them, but I can tell you that many of those plants go into emersed setups, where there are no shrimps, or even fish.
Also pesticides do sometimes stay on the leaves and stems of plants, rinsing is not enough.
Also note that many of the pesticides present on those plants, if any, may not be just damaging to the shrimp and fish, they can also be damaging to your own health, as you put your hands on the tank, some tanks are open top and you may have kids at home.
So let this be a lesson to all, although many may know it already, you got shrimp and expensive fish, buy safely. What do I mean buy safely? Buy Tropica, Aquafleur, and from traders that support the forum, or people that you know.
Again, I buy from the supplier in question, but I take precautions.
I have lost shrimp from simply touching the cat, that had a flea collar, and not washing the hands when working on the tank, now imagine if I had placed a suspect plant on the tank?