Angel Fish Death

[Ashlyn]

I've had an established tank with 5 angelfish and other community fish for almost a year now, with only 1 casualty, until this.  A few days ago a woke up to one of the angel fish dead in the center of tank. I thought maybe it just had something wrong internally and went about my day after testing the water and seeing just a slight ammonia rise. Today I got home from work, and its buddy who was always with it in the back also turned up... 

I retested everything. Everything was fine, nitrates are the only thing that seem a little high at 15-20. I tested the tap water also, tap tested at 10. I'm not sure why they both passed, but all the other fish seem to be doing fine. If anyone has any ideas on why they might have died, I'd be intrested in hearing your thoughts. 

[Colu]

Have you resently done a water change if so did you add a water dechlorinator like prime 

[Atomicfish]

How big is the tank? Angels are generally pretty hardy and can also be very territorial.