Hi, sorry for the late reply. I went to one of my LFShops and got a spin test on my water, the results are below.
Since then I have used additives to increase my Gh to 6 and Kh to 4. There has been no further deaths thankfully.
The shop seemed to think that the Fluval stratum was the culprit and suggested changing it for just plain gravel. I may do this.
Regarding my yellow cherry tank the Gh is 10 and the Kh is 4. I don't know if that's excessive, but they population of yellows has grown substantially, I see shrimplets everywhere.
Hopefully this will be the end of the problem, thanks all for your help and taking the time to reply.
Ouch the picture was to large to upload but the readings were as follows.
Ph 7.4
Gh 77
Alk 31
Phos 0.0
Am 0.0
Nitrite 0.0
Nitrate 2
This is how the spin test display lists parameters.