I think you probably right there, unfortunatly my last one died today..
I went back to the fish shop today after speaking to them on the phone and took a sample of water and the last dead fish. The guy at the shop checked it over and didn't know the cause of death, bit of a mystery really. He tested the water in front of me and everything was fine, it was good to get a second opinion. Also very kindly he gave me 3 corydora sterbai to make up for my loss, their quite small at the moment and look sooo cute .. I'll be keeping a close eye on these little guys, but fingers crossed the same thing won't happen again...
Must have been a bad bunch of minnows. I actually cycled my tank using minnows and they were fine, and your water was much better. Hopefully your next batch will be hardier if you stick to minnows.