Hi Clownfish,
In fact the tank has been up and running for 3 years, so I dun think its the NTS. I am unable to vouch for the accuracy of the measurements of the water parameters, but I dun think its way off.
I've kept leathers, zoos and mushrooms b4 without much problem, so I think it must be something wrong in the tank. It is really fustrating at the rate they died off. There is also sufficient current for the leather (an additional pump). I will just wait and see, I guess. Thank you so much!