Hans,
Frequent water changes are critical to keeping healthy happy fish. Really, most fish keepers recommend 25-50% water change per week in well stocked tanks...
So, it has been months and then you did one. After several months, nitrates and other Total Dissolved Solids build up in your tank - sometimes called Old Tank Syndrome. Then when the water finally gets cleaned up a bit, the fish have become so used to the dirty water that they have real trouble adjusting to the new stuff, even though the water is better.
Do you have test kits for ammonia, nitrite and nitrate? Lets start there and see how your water is. Post results when you have them.
I'd also start doing 10% water change per day - in both your tanks if you haven't been keeping up with the other one either. Hopefully your angel will make it through.
Cathy