I took some water to 2 lfs today and they agreed with my readings, the ammonia, nitrite, nitrate and ph are all fine so that implies that I've at least got that right. The tap water is crap around here though, apparently quite hard. I might get some peat granuals to add to the filter to help the fish a little.
Oh yes, I found a half eaten cardinal today too. Not sure if he died or was attacked. I've kept a close eye on the tank all day but witnessed no further unpleasant events.
I'm learning not to trust lfs advice. The latest one I've been going to have been telling me how friendly gouramis are. You guys and another lfs totally contradict their advice. I think I'll ask here first in future.