Sorry to hear you lost the fish
As Diana said, it had obviously already gotten to the point of no return. From what you've posted, the test results do seem fine and so does everything else you are doing.
You mention a previous loach going down with an illness, how did you treat this? Did you treat in a separate tank or in the same tank? What medication did you use? Also, does your tap water contain chlorine or chloramine? If it does, do you use a dechlorinator that can deal with both chemicals when doing your water changes?
It may be useful to take a full set of water parameters for your tap water, quite often a water company will chemically adjust the tap water so that it reaches the tap at a certain pH (helps with pipe corrosion and expected taste). When adjusted water is sufficiently aerated it usually reverts to it's pre-treated parameters. This may just have an effect as you do large water changes only once a week - if your water changes were more frequent, then a 50% water change would have much less of an impact, the severity of which is dependant upon the differences between the chemistry of your tap water and tank water; I don't think this is the case but, it helps to eliminate as much as you possibly can.