My Angelfish Died

[DragonLass]

My angelfish died fairly suddenly overnight. She seemed fine the day before & was behaving normally.
All other fish in the tank are fine.
So I'm a bit puzzled as to the possible cause. My water stats are good, I did a check once I took her out.
Zero ammonia & Nitrite, & Nitrate ~10. I had just done a 50% water change 2 days before.
There was no visible sign of any white spot/ich/fin rot etc.

Only thing I can think of, is it possible it could have been her diet to blame? As her diet seemed to almost entirely consist of platy fry, as much as I tried to get her to eat some flake and/or veggies, she was always more interested in probing the gravel for fry.

Otherwise, I have no idea. If anyone has any suggestions of things to look for in my other fish, to make sure they don't suffer the same fate?

 

[Torrean]

I think it may have been the water change. Was the temp. relatively close to the tank temp. Is the tap PH very close to the tank PH? Do you use a dechlorinator? What is the temp and PH of the tank. Sometimes massive water changes +40% can change the chemistry in the tank. I'd expect some sort of disease to show though.

edit: I suggest 25% water changes when your NA reaches 50 once you monitor your NA for a while you can set up a maintenance regime to see how often you need to do water changes. If you don't want to deal with that then 10-25% a week or 25% twice a month should be fine as long as your not overstocked.