Damned fish keep dying

Lost another two danios today. Well actually one isn't dead, but she doesn't look very happy so I pulled her out to let her rest in peace (the angels were picking at her fins in the main tank). But then I could only see three remaining, I couldn't find the other one. I just found it, floating upside down amongst some plants :(

So now I only have three Danios left, and I suspect they will start dying off soon too :(

Danios are supposed to be hardy fish, able to cope with less-than-ideal water quality. And yet they're the only casualties I've had since I setup my tanks.

Not happy Jan!