Tom,
Got your Pm... I would recommend putting the fish back with the group. They generally will do better in a group and if that one turns out to be smaller, it may just have been fishes own limitations.
I'd not beat yourself up on the water changes.. Missing a water change here and there isn't go to be an issue., and I honestly don't think that alone was the issue. But if you get busy and slack in general maintenance, like fishfood and waste removal, or missing warning signs..then yes.. Thats probably your smoking gun. You could do 100 % water changes a day and still lose fish. Most likely there were multiple factors that occurred, slacking on the maintenance was likely one, but you can never real Know. The important thing is the increase water changes are making the group act more healthy and normal.. so now you know that the water quality was probably one factor. So keep that quality up there.
Hope that helps,
al